MOVEMENT OF UNDERGROUND WATER

Availability of water supplies is determined by the movement or flow of water as well as by its distribution and amount. The natural flow of water underground is caused by gravity in the larger openings, but in the smaller openings adhesion and capillarity are also important forces.

Of all the water falling on the surface, some may not go below the surface at all but may immediately evaporate or join the runoff—that is, the surface streams. Another part may penetrate a little distance into the zone of weathering and then join the runoff. Of the water which reaches the zone of saturation, a part may soon come to the surface in low areas and join the runoff, and a part may penetrate deeply.

Above the zone of saturation gravity carries the water downward in devious courses until it reaches the water table. Thereafter its course is determined largely by the lowest point of escape from the water table. In other words, the water table is an irregular surface; and under the influence of gravity the water tends to move from the high to the low points of this surface. Between the point of entrance and the point of escape from the water table, the water follows various courses, depending upon the porosity and the openings in the rocks. In general it fills all of the available openings, and uses the entire available cross section in making its progress from one point to another. The difference in height or the "head" between the point of entrance and the point of escape, together with the porosity of the rock and other factors, determine the general speed of its movement (see p. 73). With equal porosity the flow is at a maximum along a line directly connecting the two points, and on more devious courses the flow is less.

The surface water first enters the ground through innumerable small openings. Soon, however, it tends to be concentrated into channels of easiest flow, with the result that in the later part of its underground course it may be much concentrated in large trunk channels. These channels may consist of joints, or frequently of very coarse and pervious beds. The sedimentary rocks as a whole contain the most voids, and therefore the largest flow and largest supply of water is often localized in them. Of the sedimentary rocks, sandstones and limestones usually contain the largest and most continuous openings, and thus afford the freest circulation for water. The voids in fine-grained shales may equal in volume those in sandstones and limestones, but the openings are so small and discontinuous that the water does not flow freely. Regardless of total amount of water, unless there are continuous openings of some size the flow may be small.

The relations of more porous rocks to containing impervious strata also profoundly affect the flow of underground water. Between impervious strata the circulation may be concentrated and vigorous within the porous bed. Where the porous bed is not so contained, the movement may be more dispersed and less vigorous locally. The inclination of the beds, of course, also affects the direction and amount of the flow.

The influence of gravity upon underground water may locally tend toward a state of equilibrium in which there is little movement. In such a case the water is substantially ponded, and moves only when tapped by artificial openings.

WELLS AND SPRINGS

Underground water becomes available for use by means of springs and through wells or bore holes. Water rises to the surface in natural springs at points where the pressure or head, due to its entrance into the ground at a higher level, is sufficient to force it to the surface after a longer or shorter underground course. The movement may be all downward and lateral to the point of escape, or it may be downward, lateral, and upward. Ordinarily, the course of spring waters does not carry them far below the surface. Heat and gases may be added beneath the surface by contact with or contributions from cooling igneous rocks. These may accelerate the upward movement of spring waters, and yield thermal and gas-charged waters, as in the springs and geysers of Yellowstone Park.

When a well is sunk to tap the underground water supply, the water may not rise in the artificial opening but may have to be lifted to the surface.

If, however, the water is confined beneath an impervious stratum and is under pressure from the water of higher areas, a well opening may simply allow it to move upward under its own pressure or head. This pressure may carry it upward only a few feet or quite to the surface or beyond, in which latter case the well is called an artesian well. The essential condition for an artesian circulation is a porous zone, inclining downward from the surface beneath an impervious stratum which tends to confine and pond the water. The water at any point in the water-bearing rock is under pressure which is more or less equivalent to the weight of the column of water determined by the difference in height between this point and the point of entrance or feeding area of the water. If the feeding area is higher than the collar of the well, the water will rise quite to the surface; if not, it will rise only part way. Capillary resistance, however, may and usually does lessen the theoretical pressure so figured.

The flow in deep artesian circulations is ordinarily a slow one. For the artesian wells of southern Wisconsin, it has been calculated that waters entering the outcrop of the southward dipping sandstone and limestone layers in the northern part of the state have required two or three hundred years to reach a point in the southern part of the state where they are tapped. Because of this slow movement, a large number of wells in any one spot may exhaust the local supply faster than it is replenished from the remainder of the formation. The drilling of additional wells near at hand in such cases does not increase the total yield, but merely divides it among a larger number of wells.

The porosity of the rocks, and therefore the flow of an artesian circulation, may in some cases be artificially increased by blasting and shattering.

COMPOSITION OF UNDERGROUND WATERS

Underground waters are never entirely free from dissolved mineral substances, and seldom are they free from suspended particles. Some waters are desired because they contain very small quantities of dissolved mineral matter. Others are prized because they have an unusually high content of certain mineral substances. In determining the deleterious or beneficial effect of dissolved substances, much depends on the purpose for which the water is to be used,—whether for drinking, washing, steam boilers, or irrigation. Near the surface underground waters may carry bacteria, as well as animal and vegetable refuse, which from a sanitary standpoint are usually objectionable. Deeper waters are more likely to lack this contamination because of filtration through rocks and soils.

The dissolved mineral substances of underground water are derived for the most part from the solution of rocks with which the waters come in contact, particularly at or near the surface. Through the agency of underground water most of the mineral and chemical changes of rocks are produced. The dissolved substances in solution at any time and place may therefore be regarded as by-products of rock alterations. Locally they may to some extent be derived from direct emanations from cooling igneous masses.

The most common mineral substances contained in waters are lime and magnesia. Less common, but abundant locally, are soda, potash, iron, and silica. Waters contain also certain acid and gaseous substances, the most common of which is carbon dioxide; and less widespread, but locally abundant, are chlorine and sulphur dioxide. Where lime and magnesia are abundant the water is ordinarily classed as a hard water. Where absent, or subordinate to soda and potash, the water is ordinarily classed as a soft water. Large amounts of the acid substances like chlorine and sulphur are detrimental for most purposes. Where there are unusual amounts of carbon dioxide or other gases present, they may by expansion cause the water to bubble.

If we were to attempt to describe and define the characteristics, with reference to dissolved mineral content and temperature, which make a given water more desirable than another, we should enter a field of the most amazing complexity and one with many surprising contradictions. For the most widespread use, the most desirable water is a cold water as free from mineral content as possible, and especially one lacking an excess of lime and magnesia which make it hard; also lacking an excess of acid constituents like sulphur dioxide, carbon dioxide, or chlorine, which give the water a taste, or which make impossible its use in boilers. Locally and for special reasons, waters of other qualities are in demand. Waters so excessively carbonated as to bubble, sulphureted waters, chlorine waters, waters high in iron, high in silica, high in potash, high in soda, or high in magnesia, or waters of high temperature, may come to be regarded as desirable. It is an interesting fact that any water with unusual taste, or unusual mineral content, or unusual temperature, is likely to be regarded as having medicinal value. Sometimes this view is based on scientific knowledge; sometimes it is an empirical conclusion based on experience; and again it may be merely superstition. In one case the desirable feature may be the presence of a large amount of carbon dioxide; in another case it may be its absence. In one case the desirable feature may be high temperature; in another case low temperature. The same combination of qualities which in a certain locality may be regarded as highly desirable, may be regarded as highly detrimental somewhere else where certain other types of waters are in vogue.

Proprietary rights and advertising have brought certain waters into use for drinking purposes which are not essentially different from more widely available waters which are not regarded as having special value. Two springs located side by side, or a spring and a deep well, whose waters have exactly the same chemical characteristics, may be used and valued on entirely different scales. Any attempt to classify mineral waters sold to the public in any scientific way discloses a most intricate and confused situation. One can only conclude that the popularity of certain waters is not based alone on objective qualities of composition, but rather on causes which lie in the fields of psychology and commerce.

The part played by sentiment in putting value on water is well illustrated by the general preference for spring waters as compared with well waters. In the public mind, "spring water" denotes water of unusual purity and of more desirable mineral content than well water. Illustrations could be cited of districts in which the surface or spring waters have a composition not different from that of the deeper well waters, and are much more likely to be contaminated because of proximity to the surface; and yet people will pay considerable sums for the spring water in preference to the cheaply available well water.

RELATION OF GEOLOGY TO UNDERGROUND WATER SUPPLY

It is obvious that a knowledge of geology is helpful in locating an underground water supply. Locally the facts may become so well known empirically that the well driller is able to get satisfactory results without using anything but the crudest geologic knowledge; but in general, attention to geologic considerations tends to eliminate failures in well drilling and to insure a more certain and satisfactory water supply.

In drilling for water, it is essential to know the nature, succession, and structure of the rocks beneath the surface in order to be able to identify and correlate them from drill samples. The mere identification of samples is often sufficient to determine whether a well has been drilled far enough or too far to secure the maximum results. In order to arrive at any advance approximation of results for a given locality, a knowledge of the general geology of the entire region may be necessary. Especially for expensive deep artesian wells it is necessary to work out the geologic possibilities well in advance. It is useless, for instance, to look for artesian water in a granite; but in an area of gently inclined strata, with alternations of porous and impervious layers, the expert may often figure with a considerable degree of certainty the depth at which a given porous stratum will be found, and the pressure under which the water will be in this particular stratum at a given point. Even the mineral content of the water may in some cases be predicted from geologic study.

One of the most obvious and immediately useful services of the geologist in most localities is the collection and preservation of well samples for purposes of identification and correlation of rock formations, and as a guide to further drilling. Failure to preserve samples has often led to useless and expensive duplication of work.

The problem of water supply in some localities is comparatively simple and easy. In other areas there is an infinite variety of geologic conditions which affect the problem, and the geologist finds it necessary to bring to bear all the scientific knowledge of any sort which can be used,—particularly knowledge in relation to the type of rock, the stratigraphy and the structure.

SURFACE WATER SUPPLIES

Where underground water is not abundant or not cheaply available, or where larger amounts of water are needed, as in large cities or for irrigation purposes, surface water is used. In general, surface waters are more likely to be contaminated by vegetable and animal matter and to require purification for drinking purposes.

Surface waters are also used for irrigation, water power, drainage, the carrying of sewage, etc. This great variety of uses brings the consideration of surface waters into many fields other than geology, but an understanding and interpretation of the geological conditions is none the less fundamental. This is evidenced by the inclusion of geologic discussions in most textbooks of hydrology, and in the reports of the Hydrographic Branch of the U. S. Geological Survey. The very fact that this important branch of governmental investigation is in a charge of the U. S. Geological Survey indicates its close relation to geology.

The principles of geology used in the study of surface waters relate chiefly to physiography (see Chapter I). It is usually necessary to know the total quantity of flow, its annual and seasonal variation, and the possible methods of equalization or concentration; the maximum quantity of flow, the variation during periods of flood, and the possibilities of reduction or control; the minimum flow and its possible modification by storage or an auxiliary supply. These questions are obviously related to the size and shape of the catchment area, the topography, the rock structure, the relation between underground flow or absorption and the runoff, and other physiographic factors. Quoting from D. W. Mead:[12]

Geological conditions are frequently of great importance in their influence on the quantity and regularity of runoff. If the geological deposits of the drainage area are highly impervious, the surface flow will receive and transmit the water into the mass only through the cracks and fissures in the rock. Pervious materials, such as sandstones, sands, gravels, and cracked or fissured rocks, induce seepage, retard runoff, and, if such deposits are underlaid with an impervious bed, provide underground storage which impounds water away from the conditions which permit evaporation, and hence tends to increase runoff and equalize flow. On the other hand, if such pervious deposits possess other outlets outside of the stream channel and drainage area, they may result in the withdrawal of more or less of the seepage waters entirely from the ultimate flow of the stream. Coarse sands and gravels will rapidly imbibe the rainfall into their structure. Fine and loose beds of sand also rapidly receive and transmit the rainfall unless the precipitation is exceedingly heavy under which conditions some of it may flow away on the surface.

Many of the highly pervious indurated formations receive water slowly and require a considerable time of contact in order to receive and remove the maximum amount.

In flat, pervious areas, rainfalls of a certain intensity are frequently essential to the production of any resulting stream flow. In a certain Colorado drainage area, the drainage channel is normally dry except after a rainfall of one-half inch or more. A less rainfall, except under the condition of a previously saturated area, evaporates and sinks through the soil and into the deep lying pervious sand rock under the surface which transmits it beyond the drainage area. Such results are frequently greatly obscured by the interference of other factors, such as temperature, vegetation, etc.

* * * * *

The natural storage of any drainage area and the possibilities of artificial storage depend principally upon its topography and geology. Storage equalizes flow, although the withdrawal of precipitation by snow or ice storage in northern areas often reduces winter flow to the minimum for the year. Both surface and sub-surface storage sometimes hold the water from the streams at times when it might be advantageously used. Storage, while essential to regulation, is not always an advantage to immediate flow conditions.

UNDERGROUND AND SURFACE WATERS IN RELATION TO EXCAVATION AND CONSTRUCTION

Scarcely more than a mention of this subject is necessary. In mining, the pumping charge is one of the great factors of cost. A forecast of the amount and flow of water to be encountered in mining is based on the geologic conditions. The same is true in excavating tunnels, canals, and deep foundations. Detailed study of the amount and nature of water in the rock and soil of the Panama Canal has been vital to a knowledge of the cause and possibilities of prevention of slides. Rock slides in general are closely related to the amount and distribution of the water content.

The importance of ground-water as a detriment in military operations was shown during the recent war in trenching and other field works. At the outset, with the possible exception of the German army, a lack of scientific study of ground-water conditions led to much unnecessary difficulty. It soon became necessary to study and map the water conditions in great detail in advance of operations. Much of this work was done by geologists (see Chapter XIX).

Geological considerations are involved in a great variety of engineering undertakings related to river and harbor improvements, dam sites, etc., mentioned in Chapter XX.

FOOTNOTES:

[12] Mead, Daniel W., Hydrology: McGraw-Hill Book Co., New York, 1919, pp. 447-448, 456.



CHAPTER VI

THE COMMON ROCKS AND SOILS AS MINERAL RESOURCES

ECONOMIC FEATURES OF THE COMMON ROCKS

Under the general heading of common rocks are included the ordinary igneous, sedimentary, and "metamorphic" rocks, and the unconsolidated clays, sands, and gravels characteristic of surface conditions, which are mined and quarried for commercial use. Soils are closely related to this group; but since they present special problems of their own, they are discussed under a separate heading at the end of the chapter. Names of the common rocks will be used with the general commercial significance given them by the United States Geological Survey in its mineral resource reports.

Because of their inexhaustible quantity and ready availability, the value of the common rock products is not large per unit of weight; but in the aggregate it ranks high among mineral products. In respect to tonnage, common rocks constitute perhaps 10 per cent of the world annual output of all mineral commodities (exclusive of water).

The greater tonnage of the common rocks is used commercially in crushed or comminuted forms for road material, for railroad ballast, and for cement, brick, concrete, and flux. In blocks and structural shapes, of less aggregate tonnage, they are used as building stone, monumental stone, paving blocks, curbing, flagging, roofing, refractory stone, and for many other building and manufacturing purposes.

The common rocks are commodities in which most countries of the globe are self-sufficing. International trade in these commodities is insignificant, being confined to small quantities of materials for special purposes, or to local movements of short distances, allowed by good transportation facilities.

The common rocks are so abundant and widespread that the conservation of raw materials is not ordinarily a vital problem. Conservational principles do apply, however, to the human energy factor required for their efficient use. In the valuation of common rocks, also, the more important factors are not the intrinsic qualities of the stones, but rather the conditions of their availability for use.

Because of bulk and comparatively low intrinsic value, the principal commercial factors in the availability of the common rocks are transportation and ease of quarrying, but these are by no means the only factors determining availability. Their mineral and chemical composition, their texture and structure, their durability, their behavior under pressure and temperature changes, and other factors enter in to important degrees. The weighting and integration of these factors, for the purpose of reaching conclusions as to the availability of particular rock materials, depend also on the purposes for which these materials are to be used. The problem is anything but simple. The search for a particular rock to meet a certain demand within certain limits of cost is often a long and arduous one. On account of the abundance and widespread distribution of common rocks and their variety of uses, there is a good deal of popular misapprehension as to their availability. Many building and manufacturing enterprises have met disastrous checks, because of a tendency to assume availability of stone without making the fullest technical investigation. Many quarrying ventures have come to grief for the same reason. It is easy to assume that, because a granite in a certain locality is profitably quarried and used, some other granite in the same locality has equal chances. However, minor differences in structure, texture, and composition, or in costs of quarrying and transportation, may make all the difference between profit and loss. Even though all these conditions are satisfactorily met, builders and users are often so conservative that a new product finds difficulty in breaking into the market. A well-established building or ornamental stone, or a limestone used for flux, may hold the market for years in the face of competition from equally good and cheaper supplies. The very size of a quarry undertaking may determine its success or failure.

GRANITE

The term granite, as used commercially, includes true granite and such allied rocks as syenite and gneiss. In fact even quartzite is sometimes called granite in commerce, as in the case of the Baraboo quartzites of Wisconsin, but this is going too far. For statistical purposes, the United States Geological Survey has also included small quantities of diorite and gabbro. The principal uses of granite are, roughly in order of importance, for monumental stone, building stone, crushed stone, paving, curbing, riprap and rubble. Thirty states in the United States produce granite, the leaders being Vermont, Massachusetts, North Carolina, Maine, Wisconsin, Minnesota, and California.

BASALT AND RELATED TYPES

Basalt and related rocks are sometimes included under the name "trap rock," which comprises,—besides typical basalt and diabase,—fine-grained diorite, gabbro, and other basic rocks, which are less common in occurrence and are similar in chemical and physical properties. The principal use of these rocks is as crushed stone for road and ballast purposes and for concrete. They are produced in some fifteen states, the leaders being New Jersey, Pennsylvania, California, and Connecticut.

LIMESTONE, MARL, CHALK

In the United States limestone is used principally as crushed stone for road material, railroad ballast, concrete, and cement, as fluxing stone for metallurgical purposes, and in the manufacture of lime. Minor uses are as building stones, paving blocks, curbing, flagging, rubble, and riprap; in alkali works, sugar factories, paper mills, and glass works; and for agricultural purposes. For the making of cement, in metallurgical fluxes, and in most of the manufacturing and agricultural uses, both limestone and lime (limestone with the CO_2 driven out by heating) are used. Lime is also extensively used in the making of mortar for building operations, in tanning leather, and in a great variety of chemical industries. The total quantity of limestone used for all purposes in the United States nearly equals that of iron ore. Nearly every state in the union produces limestone, but the more important producers are Pennsylvania (where a large amount is used for fluxing), Ohio, Indiana, New York, Michigan, and Illinois.

Closely associated with limestone in commercial uses, as well as in chemical composition, is calcareous marl, which is used extensively in the manufacture of Portland cement.

Chalk is a soft amorphous substance of the same composition as limestone. The main uses of chalk are as a filler in rubber, and as a component of paint and putty. It is also used for polishing. The principal producers of this commodity are England, Denmark, and France, and the chief consumer is the United States. The United States depends upon imports for its supply of chalk for the manufacture of whiting. Before the war two-thirds came from England and a third from France. During the war importation was confined to England, with a small tonnage from Denmark. No deposits of domestic chalk have been exploited commercially. A somewhat inferior whiting, but one capable of being substituted for chalk in most cases, is manufactured from the waste fine material of limestone and marble quarries.

MARBLE

Marble is limestone which has been coarsely recrystallized by metamorphism. The marble of commerce includes a small quantity of serpentine as quarried and sold in Massachusetts, California, Maryland, Pennsylvania, and Vermont, and also a small amount of so-called onyx marble or travertine obtained from caves and other deposits in Kentucky and other states. The principal uses of marble are for building and monumental stones. Of the twenty-two states producing marble, the leaders are Vermont, Georgia, and Tennessee.

A small amount of marble of special beauty, adapted to ornamental purposes, is imported from European countries, especially from Italy. Marble imports from Italy constitute about two-thirds, both in tonnage and value, of all stone imported into the United States.

SAND, SANDSTONE, QUARTZITE (AND QUARTZ)

Sand is composed mainly of particles of quartz or silica, though sometimes feldspar and other minerals are present. Sandstones are partially cemented sands. Quartzites are completely cemented sands. To some extent these substances are used interchangeably for the same purposes.

The principal uses of sand in order of commercial totals are for building purposes—for mortar, concrete, sand-lime brick, etc.,—as molding sand in foundries, as a constituent of glass, in grinding and polishing, in paving, as engine sand, as fire or furnace sand, in the manufacture of ferrosilicon (a steel alloy), and in filters. Reference is made to sand as an abrasive and in the manufacture of steel in Chapters XIII and IX. Almost every state produces some sand, but for some of the more specialized uses, such as glass sand, molding sand, and fire or furnace sand, the distribution is more or less limited. The United States Geological Survey has collected information concerning the distribution of various kinds of sand and gravel, and serves a very useful function in furnishing data as to supplies of material for particular purposes. Fine molding sands have been imported from France, but during the war domestic sources in New York and Ohio were developed sufficiently to meet any requirements.

The sandstone of commerce includes the quartzites of Minnesota, South Dakota, and Wisconsin, and the fine-grained sandstones of New York, Pennsylvania, and elsewhere, known to the trade as "bluestone." In Kentucky most of the sandstone quarried is known locally as "freestone." The principal uses of sandstone are for building stone, crushed stone, and ganister (for silica brick and furnace-linings). Other uses are for paving blocks, curbing, flagging, riprap, rubble, grindstones, whetstones, and pulpstones (see also Chapter XIII). Sandstone is sometimes crushed into sand and is used in the manufacture of glass and as molding-sand. Most of the states of the union produce sandstone, the principal producers being Pennsylvania, Ohio, and New York.

"SAND AND GRAVEL"

Where sand is coarse and impure and mixed with pebbles, it is Ordinarily referred to as "sand and gravel." For sand and gravel the principal uses are for railroad ballast, for road building, and for concrete. Sand and gravel are produced in almost every state in the union, the largest producers being Pennsylvania, Ohio, Illinois, New Jersey, and North Carolina.

CLAY, SHALE, SLATE

Shale is consolidated clay, usually with a fine lamination due to bedding. Slate is a more dense and crystalline rock, produced usually by the anamorphism of clay or shale under pressure, and characterized by a fine cleavage which is usually inclined to the sedimentary bedding.

Clays are used principally for building and paving brick and tile, sewer-pipe, railroad ballast, road material, puddle, Portland cement, and pottery. Clay is mined in almost every state. Ohio, Pennsylvania, New Jersey, and Illinois have the largest production. There has been a considerable importation of high-grade clays, principally from England, for special purposes—such as the filling and coating of paper; the manufacture of china, of porcelain for electrical purposes, and of crucibles; and for use in ultramarine pigments, in sanitary ware, in oilcloth, and as fillers in cotton bleacheries. War experience showed the possibility of substitution of domestic clays for most of these uses; but results were not in all cases satisfactory, and the United States will doubtless continue to use imported clays for some of these special purposes.

Shales, because of their thinly bedded character and softness, are of no value as building stones, but are used in the manufacture of brick, tile, pottery, and Portland cement.

Slates owe their commercial value primarily to their cleavage, which gives well-defined planes of splitting. The principal uses are for roofing and, in the form of so-called mill stock for sanitary, structural, and electrical purposes. Small amounts are used for tombstones, roads, slate granules for patent roofing, school slates, blackboard material, billiard table material, etc. The color, fineness of the cleavage, and size of the flakes are the principal features determining the use of any particular slate. Ten states produce slate, the principal production coming from Pennsylvania and Vermont.

THE FELDSPARS

Feldspars are minerals, not rocks, but mention of them is made here because, with quartz, they make up such an overwhelming percentage of earth materials. It is estimated that the feldspars make up 50 per cent of all the igneous rocks and 16 per cent of the sedimentary rocks. As the igneous rocks are so much more abundant than the sedimentary rocks, the percentage of feldspars in the earth approaches the former rather than the latter figure. In most rocks feldspar is in too small grains and is too intimately associated with other minerals to be of commercial importance; in only one type of rock, pegmatite, which is an igneous rock of extremely coarse and irregular texture, are the feldspar crystals sufficiently large and concentrated to be commercially available.

Feldspar is used principally in the manufacture of pottery, china ware, porcelain, enamel ware, and enamel brick and tile. In the body of these products it is used to lower the fusing point of the other ingredients and to form a firm bond between their particles. Its use in forming the glaze of ceramic products is also due to its low melting point. A less widespread use of feldspar is as an abrasive (Chapter XIII). One of the varieties of feldspar carries about 15 per cent of potash, and because of the abundance of the mineral there has been much experimental work to ascertain the possibility of separating potash for fertilizer purposes; but, because of cost, this source of potash is not likely for a long time to compete with the potash salts already concentrated by nature.

Feldspar is mined in eleven states, but the important production comes from North Carolina and Maine. The United States also imports some feldspar from Canada.

HYDRAULIC CEMENT (including Portland, natural, and Puzzolan cements)

Cement is a manufactured product made from limestone (or marl) and clay (or shale). Sometimes these two kinds of substances are so combined in nature (as in certain clayey limestones) that they are available for cement manufacture without artificial mixing. It is not our purpose in this volume to discuss manufactured products; but the cement industry involves such a simple transformation of raw materials, and is so closely localized by the distribution of the raw materials, that a mention of some of its outstanding features seems desirable.

Hydraulic cement is used almost exclusively as a structural material. It is an essential ingredient of concrete. Originally used chiefly for the bonding of brick and stone masonry and for foundation work, its uses have grown rapidly, especially with the introduction of reinforced concrete. It is being used in the construction of roads, and its latest use is in ship construction.

With the exception of satisfactory fuels, the raw materials required for the manufacture of cement are found quite generally throughout the world. While practically all countries produce some cement, much of it of natural grade, only the largest producers make enough for their own requirements and as a result there is a large world movement of this commodity. The world trade is chiefly in Portland cement.

Next to the United States, the producing countries having the largest exportable surplus of cement in normal times are Germany and Great Britain. France and Belgium were both large producers and exporters before the war, but the war greatly reduced their capacity to produce for the time being. Sweden, Denmark, Austria, Japan, and Switzerland all produce less extensively but have considerable surplus available for export. Italy and Spain have large productions, which are about sufficient for their own requirements. Holland and Russia import large amounts from the other European countries. The far eastern trade absorbs the excess production of Japan. In South Africa and Australasia, production nearly equals demand. In Canada, although the industry has been growing very rapidly, the demand still exceeds production. In South and Central America, Mexico and the West Indies, the demand is considerable and will probably increase; production has thus far been insufficient. Several modern mills are either recently completed or under construction in these countries, and concessions have been granted for several others. These new mills are largely financed by American capital.

The United States is the largest single producer of cement in the world, its annual production being about 45 per cent of the world's total. Domestic consumption has always been nearly as great as the production, and exports have usually not exceeded 4 per cent of the total shipments from the mills. South and Central America offer fields for exportation of cement from the United States.

GEOLOGIC FEATURES OF THE COMMON ROCKS

To describe the geologic features of the common rocks used in commerce would require a full treatise on the subject of geology. These are the bulk materials of the earth and in them we read the geologic history of the earth. In preceding chapters a brief outline has been given of the relative abundance of the common earth materials and of the processes producing them. In comparison, the metalliferous deposits are the merest incidents in the development of this great group of mineral resources.

In this section reference will be made only to a few of the rock qualities and other geologic features which require first attention in determining the availability of a common rock for commercial use. The list is very fragmentary, for the reason that the uses are so many and so varied that to describe all the geologic features which are important from the standpoint of all uses would very soon bring the discussion far beyond the confines of a book of this scope.[13]

BUILDING STONE

For building stones, the principal geologic features requiring attention are structure, durability, beauty, and coloring.

The structures of a rock include jointing, sedimentary stratification, and secondary cleavage. Nearly all rocks are jointed. The joints may be open and conspicuous, or closed and almost imperceptible. The closed joints or incipient joints cause planes of weakness, known variously as rift, grain, etc., which largely determine the shapes of the blocks which may be extracted from a quarry. Where properly distributed, they may facilitate the quarrying of the stone. In other cases they may be injurious, in that they limit the size of the blocks which can be extracted and afford channels for weathering agents. Some rocks of otherwise good qualities are so cut by joints that they are useless for anything but crushed stone. The bedding planes or stratification of sedimentary rocks exercise influences similar to joints, and like joints may be useful or disadvantageous, depending on their spacing. The secondary cleavage of some rocks, notably slates, enables them to be split into flat slabs and thus makes them useful for certain purposes.

Proper methods of extraction and use of a rock may minimize the disadvantageous effects of its structural features. The use of channelling machines instead of explosives means less shattering of the rock. By proper dressing of the surface the opening of small crevices may be avoided. Stratified rocks set on bed, so that the bedding planes are horizontal, last longer than if set on edge.

The durability of a rock may depend on its perviousness to water which may enter along planes of bedding or incipient fracture planes, or along the minute pore spaces between the mineral particles. The water may cause disastrous chemical changes in the minerals and by its freezing and thawing may cause splitting. For this reason, the less pervious rocks have in general greater durability than the more pervious. Highly pervious rocks used in a dry position or in a dry climate will last longer than elsewhere.

Durability is determined also by the different coefficients of expansion of the constituent minerals of the rock. Where the minerals are heterogeneous in this regard, differential stresses are more likely to be set up than where the minerals are homogeneous. Likewise a coarse-textured rock is in general less durable than a fine-textured one. Expansion and contraction of a stone under ordinary temperature changes, and also under fire and freezing, must necessarily be known for many kinds of construction.

Minerals resist weathering to different degrees, therefore the mineral composition of a rock is another considerable factor in determining its durability. Where pyrite is present in abundance it easily weathers out, leaving iron-stained pits and releasing sulphuric acid which decomposes the rock. Abundance of mica, especially where segregated along the stratification planes, permits easy splitting of the rock under weathering. Likewise the mica often weathers more quickly than the surrounding minerals, giving a pitted appearance; in marbles and limestones its irregular occurrence may spoil the appearance. Flint or chert in abundance is deleterious to limestones and marbles, because, being more resistant, it stands out in relief on the weathered surface, interferes with smooth cutting and polishing, and often causes the rock to split along the lines of the flint concretions. Abundance of tremolite may also be disadvantageous to limestones and marbles, because it weathers to a greenish-yellow clay and leaves a pitted surface.

The crushing strength of a rock has an obvious relation to its structural uses. The rock must be strong enough for the specified load. Most hard rocks ordinarily considered for building purposes are strong enough for the loads to which subjected, and this factor is perhaps ordinarily less important than the structural and mineral features already mentioned.

It is often necessary to know the modulus of elasticity and other mechanical constants of a rock, as in cases where it is to be combined with metal or other masonry or to be subjected to exceptional shock.

The beauty and coloring of a rock are its esthetic rather than its utilitarian features. They are particularly important in the construction of buildings and monuments for public or ornamental purposes.

CRUSHED STONE

The largest use of rock or stone is in the crushed form for road building, railway embankments, and concrete, and the prospect is for largely increased demands for such uses in the future. For the purpose of road building, it is necessary to consider a stone's resistance to abrasion, hardness, toughness, cementing value, absorption, and specific gravity. Limestone cements well, but in other qualities it is not desirable for heavy traffic. Shales are soft and clayey, and grind down to a mass which is dry and powdery, and muddy in wet weather. Basalt and related rocks resist abrasion, and cement well. Granites and other coarse-grained igneous rocks do not cement well and are not resistant to abrasion. Many sandstones are very hard and brittle and resist abrasion, but do not cement.

The application of geology on a large scale to the study of sources and qualities of crushed stone is now being required in connection with the great state and national projects of highway building. This work is by no means confined to a mere testing of the physical qualities of road-building materials found along the proposed route, but includes a careful study of their geologic occurrence, distribution, and probable amounts. In certain of the northern states specialists in glacial geology are preferred for this purpose.

STONE FOR METALLURGICAL PURPOSES

The use of limestone and other rock for metallurgical fluxes is dependent very largely on chemical composition. Comparatively few limestones are sufficiently pure for this purpose. For furnace linings, the quartzite or ganister must be exceptionally pure. The field search for rocks of the necessary composition has required geologic service.

CLAY

For a variety of uses to which clay is put, it is necessary to know its degree of plasticity, tensile strength, shrinkage (both under air and fire), fusibility, color, specific gravity, and chemical properties. The testing of clay for its various possible uses is a highly specialized job, usually beyond the range of a geologist, although certain geologists have been leaders in this type of investigation. More commonly within the range of a geologist are questions concerning origin, field classification, distribution, quantities, and other geologic conditions affecting quality and production.

Clay originates from the weathering of common rocks containing silicates, by pretty well understood weathering processes (see Chapter II). It may remain in place above the parent rock, or may be transported and redeposited, either on land or under water, by the agencies of air, water, and ice. The kind of parent rock, the climatic conditions and nature of the weathering, and the degree of sorting during transportation, all determine the composition and texture of the resulting clay,—with the result that a classification on the basis of origin may indicate the broad group characteristics which it is desirable to know for commercial purposes. For instance, residual clays from the weathering of granite may be broadly contrasted with residual clays formed by the weathering of limestone, and both differ in group characteristics from clays in glacial deposits. Classification according to origin also may be useful in indicating general features of depth, quantity, and distribution. However, a genetic classification of clays is often not sufficient to indicate the precise characteristics which it is necessary to know in determining their availability for narrow and special technical requirements. Furthermore, clays suitable for certain commercial requirements may be formed in several different ways, and classification based on specific qualities may therefore not correspond at all to geologic classification based on origin.

Geologists have been especially interested in the causes of plasticity of clay and in its manner of hardening when dried. In general these phenomena have been found to be due to content of colloidal substances of a clayey nature, which serve not only to hold the substance together during plastic flow but to bind it during drying. The part played by colloids in the formation of clays, as well as of many other mineral products, is now a question which is receiving intensive study.

The same processes which produce clay also produce, under special conditions, iron ores, bauxites, the oxide zones of many sulphide ore bodies, and soils, all of which are referred to on other pages.

LIMITATIONS OF GEOLOGIC FIELD IN COMMERCIAL INVESTIGATION OF COMMON ROCKS

In general the qualities of the earth materials which determine their availability for use are only to a minor extent the qualities which the geologist ordinarily considers for mapping and descriptive purposes. The usual geological map and report on a district indicate the distribution and general nature of the common rocks, and also the extent to which they are being used as mineral resources. Seldom, however, is there added a sufficiently precise description, for instance of a clay, to enable the reader to determine which, if any, of the many different uses the material might be put to. The variety of uses is so great, and the technical requirements for different purposes are so varied and so variable, that it is almost impossible to make a description which is sufficiently comprehensive, and at the same time sufficiently exact, to give all the information desired for economic purposes. If the geologist is interested in disclosing the commercial possibilities in the raw materials of an area, he may select some of the more promising features and subject them to the technical analysis necessary to determine their availability for special uses. In this phase of his work he may find it necessary to enlist the cooperation of skilled technicians and laboratories in the various special fields. The problem is simplified if the geologist is hunting for a particular material for a specific purpose, for then he fortifies himself with a knowledge of the particular qualities needed and directs his field and laboratory study accordingly.

Too often the geologist fails to recognize the complexity and definiteness of the qualities required, and makes statements and recommendations on the use of raw materials based on somewhat general geologic observations. On the other hand, the engineer, or the manufacturer, or the builder often goes wrong and spends money needlessly, by failing to take into consideration general geologic features which may be very helpful in determining the distribution, amount, and general characters of the raw materials needed.

It is difficult to draw the line between the proper fields of the geologist and those of the engineer, the metallurgist, and other technicians. It is highly desirable that the specialist in any one of these fields know at least of the existence of the other fields and something of their general nature. Too often his actions indicate he is not acutely conscious even of the existence of these related branches of knowledge. The extent and detail to which the geologist will familiarize himself with these other fields will of course vary with his training and the circumstances of his work. Whatever his limit is, it should be definitely recognized; his work should be thorough up to this limit and his efforts should not be wasted in fields which he is not best qualified to investigate.

These remarks apply rather generally to mineral resources, but they are particularly pertinent in relation to the common rock materials which the geologist is daily handling,—for he is likely to assume that he knows all about them and that he is qualified to give professional advice to industries using them. In connection with metallic resources, the metallurgical and other technical requirements are likely to be more definitely recognized and the lines more sharply drawn, with the result that the geologist is perhaps not so likely to venture into problems which he is not qualified to handle.

The limits to geologic work here discussed are not necessarily limits separating scientific from non-scientific work. The study and determination of the qualities of rocks necessary for commercial purposes is fully as scientific as a study of the qualities commonly considered in purely geologic work, and the results of technical commercial investigations may be highly illuminating from a purely geological standpoint. When a field of scientific endeavor has been established by custom, any excursion beyond traditional limits is almost sure to be regarded by conservatives in the field as non-scientific, and to be lightly regarded. The writer is fully conscious of the existence of limits and the necessity for their recognition; but he would explain his caution in exceeding these limits on the ground of training and effectiveness, rather than on fear that he is becoming tainted with non-scientific matters the moment he steps beyond the boundaries of his traditional field.

SOILS AS A MINERAL RESOURCE

Soils are not ordinarily listed as mineral resources; but as weathered and altered rock of great economic value, they belong nearly at the head of the list of mineral products.

ORIGIN OF SOILS

Soil originate from rocks, igneous, sedimentary, and "metamorphic" by processes of weathering, and by the mixing of the altered mineral products with decayed plant remains or humus. The humus averages perhaps 3 or 4 per cent of the soil mass and sometimes constitutes as much as 75 per cent. Not all weathered rock is soil in the agricultural sense. For this purpose the term is mainly restricted to the upper few inches or feet penetrated by plant roots.

The general process of soil formation constitutes one of the most important phases of katamorphism—the destructive side of the metamorphic cycle, described in Chapter II. Processes of katamorphism or weathering, usually accompanied by the formation of soils, affect the surface rocks over practically all the continental areas.

The weathering of a highly acid igneous rock with much quartz produces a residual soil with much quartz. The weathering of a basic igneous rock without quartz produces a clay soil without quartz, which may be high in iron. Where disintegration has been important the soil contains an abundance of the original silicates of the rock, and less of the altered minerals.

The production of soil from sedimentary rocks involves the same processes as alter igneous rocks; but, starting from rocks of different composition, the result is of course different in some respects. Sandstones by weathering yield only a sandy soil. Limestones lose their calcium carbonate by solution, leaving only clay with fragments of quartz or chert as impurities. A foot of soil may represent the weathering of a hundred feet of limestone. Shales may weather into products more nearly like those of the weathering of igneous rocks. Silicates in the shales are broken down to form clay, which is mixed with the iron oxide and quartz.

In some localities the soil may accumulate to a considerable depth, allowing the processes of weathering to go to an extreme; in others the processes may be interrupted by erosion, which sweeps off the weathered products at intermediate stages of decomposition and may leave a very thin and little decomposed soil.

Soils formed by weathering may remain in place as residual soils, or they may be transported, sorted, and redeposited, either on land or under water. It is estimated by the United States Bureau of Soils[14] that upward of 90 per cent of the soils of the United States which have been thus far mapped owe their occurrence and distribution to transportation by moving water, air, and ice (glaciers), and that less than 10 per cent have remained in place above their parent rock. Glaciers may move the weathered rock products, or they may grind the fresh rocks into a powder called rock flour, and thus form soils having more nearly the chemical composition of the unaltered rocks. Glacial soils are ordinarily rather poorly sorted, while wind and water-borne soils are more likely to show a high degree of sorting.

The character of a transported soil is less closely related to the parent rock than is that of a residual soil, because the processes of sorting and mixture of materials from different sources intervene to develop deposits of a nature quite different from residual soils; but even transported soil may sometimes be traced to a known rock parentage.

Where deposited under water, soil materials may be brought above the water by physiographic changes, and exposed at the surface in condition for immediate use. Or, they may become buried by other sediments and not be exposed again until after they have been pretty well hardened and cemented,—in which case they must again undergo the softening processes of weathering before they become available for use. Where soils become buried under other rocks and become hardened, they are classed as sedimentary rocks and form a part of the geologic record. Many residual and transported soils are to be recognized in the geologic column; in fact a large number of the sedimentary rocks ordinarily dealt with in stratigraphic geology are really transported soils.

The development of soils by weathering should not be regarded as a special process of rock alteration, unrelated to processes producing other mineral products. Exactly the same processes that produce soils may yield important deposits of iron ore, bauxite, and clay, and they cause also secondary enrichment of many metallic mineral deposits. For instance the weathering of a syenite rock containing no quartz, under certain conditions, as in Arkansas, results in great bauxite deposits which are truly soils and are useful as such,—but which happen to be more valuable because of their content of bauxite. The weathering of a basic igneous rock, as in Cuba, may produce important residual iron ore deposits, which are also used as soils. Weathering of ferruginous limestone may produce residual iron and manganese ores in clay soils.

COMPOSITION OF SOILS AND PLANT GROWTH

The mineral ingredients in soils which are essential for plant growth include water, potash, lime, magnesia, nitrates, sulphur, and phosphoric acid—all of which are subordinate in amount to the common products of weathering (pp. 20-22, 23-24). Of these constituents magnesia is almost invariably present in sufficient quantity; while potash, nitrates, lime, sulphur, and phosphoric acid, although often sufficiently abundant in virgin soil, when extracted from the soils by plant growth are liable to exhaustion under ordinary methods of cultivation, and may need to be replenished by fertilizers (Chapter VII). Some soils may be so excessively high in silica, iron, or other constituents, that the remaining constituents are in too small amounts for successful plant growth.

Even where soils originally have enough of all the necessary chemical elements, one soil may support plant growth and another may not, for the reason that the necessary constituents are soluble and hence available to the plant roots in one case and are not soluble in the other. Plainly the mineral combinations in which the various elements occur are important factors in making them available for plant use. Similarly a soil of a certain chemical and mineralogical composition may be fruitful under one set of climatic conditions and a soil of like composition may be barren at another locality—indicating that availability of constituents is also determined by climatic and other conditions of weathering. Even with the same chemical composition and the same climatic conditions, there may be such differences in texture between various soils as to make them widely different in yield.

The unit of soil classification is the soil type, which is a soil having agricultural unity, as determined by texture, chemical character, topography, and climate. The types commonly named are clay, clay loam, silt loam, loam, fine sandy loam, sandy loam, fine sand, and sand. In general the soil materials are so heterogeneous and so remote from specific rock origin, that in such classification the geologic factor of origin is not taken into account. More broadly, soils may be classified into provinces on the basis of geography, similar physiographic conditions, and similarity of parent rocks; for instance, the soils of the Piedmont plateau province, of the arid southwest region, of the glacial and loessal province, etc. In such classification the geologic factors are more important. Soils within a province may be subdivided into "soil series" on the basis of common types of sub-soils, relief, drainage, and origin.

USE OF GEOLOGY IN SOIL STUDY

While the desirability of particular soils is related in a broad way to the character of the parent rocks, and while by geologic knowledge certain territories can be predicated in advance as being more favorable than others to the development of good soils, so many other factors enter into the question that the geologic factor may be a subordinate one. A soil expert finds a knowledge of geology useful as a basis for a broad study of his subject; but in following up its intricacies he gives attention mainly to other factors, such as the availability of common constituents for plant use, the existence and availability of minute quantities of materials not ordinarily regarded as important by the geologist, the climatic conditions, and the texture. As the geologic factors are many of them comparatively simple, much of the expert work on soils requires only elementary and empirical knowledge of geology. The geologist, although he may understand fully the origin of soils and may indicate certain broad features, must acquire a vast technique not closely related to geology before he becomes effective in soil survey work and diagnosis.

For these reasons the mapping and classification of soils, while often started by geologists of state or federal surveys, have in their technical development and application now passed largely into the hands of soil experts in the special soil surveys affiliated with the U. S. Department of Agriculture and with agricultural colleges.

FOOTNOTES:

[13] A good summary of this subject may be found in Engineering Geology, by H. Ries and T. L. Watson, Wiley and Sons, 2d ed., 1915.

[14] Marbut, Curtis F., Soils of the United States: Bull. 96, Bureau of Soils, 1913, p. 10.



CHAPTER VII

THE FERTILIZER GROUP OF MINERALS

GENERAL COMMENTS

Soils are weathered rock more or less mixed with organic material. The weathering processes forming soils are in the field of geologic investigation, but the study of soils in relation to agriculture requires attention to texture and to several of their very minor constituents which have little geologic significance. Soil study has therefore become a highly specialized and technicalized subject,—for which a geological background is essential, but which is usually beyond the range of the geologist. To supply substances which are deficient in soils, however, requires the mining, quarrying, or extraction of important mineral resources, and in this part of the soil problem the geologist is especially interested.

Soils may be originally deficient in nitrates, phosphates, or potash; or the continued cropping of soils may take out these materials faster than the natural processes of nature supply them. In some soils there are sufficient phosphates and potash to supply all plant needs indefinitely; but the weathering and alteration processes, through which these materials are rendered soluble and available for plant life, in most cases are unable to keep up with the depletion caused by cropping. A ton of wheat takes out of the soil on an average 47 pounds of nitrogen, 18 pounds of phosphoric acid, 12 pounds of potash. On older soils in Europe it has been found necessary to use on an average 200 pounds of mixed mineral fertilizers annually per acre. On the newer soils of the United States the average thus far used has been less than one-seventh of this amount. The United States has thus far been using up the original materials stored in the soil by nature, but these have not been sufficient to yield anything like the crop output per acre of the more highly fertilized soils of Europe.

In addition to the nitrates, phosphates, and potassium salts, important amounts of lime and sulphuric acid, and some gypsum, are used in connection with soils. Lime is derived from crushed limestone (pp. 82-83), and is used primarily to counteract acidity or sourness of the soil; it is, therefore, only indirectly related to fertilizers. Sulphuric acid is used to treat rock phosphates to make them more soluble and available to plant life. It requires the mining of pyrite and sulphur. Gypsum, under the name of "land-plaster," is applied to soils which are deficient in the sulphur required for plant life; increase in its use in the future seems probable. There are also considerable amounts of inert mineral substances which are used as fillers in fertilizers to give bulk to the product, but which have no agricultural value. The proportions of the fertilizer substances used in the United States are roughly summarized in Figure 4.

The United States possesses abundant supplies of two of the chief mineral substances entering into commercial fertilizers,—phosphate rock and the sulphur-bearing materials necessary to treat it. For potash the United States is dependent on Europe, unless the domestic industry is very greatly fostered under protective tariff. For the mineral nitrates the United States has been dependent on Chile, and because of the cheapness of the supply will doubtless continue to draw heavily from this source. However, because of the domestic development of plants for the fixation of nitrogen from the air, the recovery of nitrogen from coal in the by-product processes, and the use of nitrogenous plants, the United States is likely to require progressively less of the mineral nitrates from Chile.

The fertilizer industry of the United States is yet in its infancy and is likely to have a large growth. Furthermore much remains to be learned about the mixing of fertilizers and the amounts and kinds of materials to be used. The importance of sulphur as a plant food has been realized comparatively recently. The use of fertilizers in the United States has come partly through education and the activity of agricultural schools and partly through advertising by fertilizer companies. The increased use of potash has been due largely to the propaganda of the German sales agents. An examination of a map showing distribution of the use of fertilizers over the country indicates very clearly the erratic distribution of the effects of these various activities. One locality may use large amounts, while adjacent territory of similar physical conditions uses little. The sudden withdrawal of fertilizers for a period of three or four years during the war had very deleterious effects in some localities, but was not so disastrous as expected in others,—emphasizing the fact that the use of fertilizers has been partly fortuitous and not nicely adjusted to specific needs.



NITRATES

ECONOMIC FEATURES

There are several sources of nitrogen for fertilizer purposes: mineral nitrates, nitrogen taken from the air by certain plants with the aid of bacteria and plowed into the soil, nitrogen taken directly from the air by combining nitrogen and oxygen atoms in an electric arc, or by combining nitrogen and hydrogen to form ammonia, nitrogen taken from the air to make a compound of calcium, carbon, and nitrogen (cyanamid), nitrogen saved from coal in the form of ammonia as a by-product of coke-manufacture, and nitrogen from various organic wastes. Nitrogen in the form of ammonia is also one of the potential products of oil-shales (p. 150). While the principal use of nitrogenous materials is as fertilizers, additional important quantities are used in ammonia for refrigerating plants, and in the form of nitric acid in a large number of chemical industries. During the war the use of nitrates was largely diverted to explosives manufacture. The geologist is interested principally in the mineral nitrates as a mineral resource, but the other sources of nitrogen, particularly its recovery from coal, also touch his field.

Almost the single source of mineral nitrates for the world at present is Chile, where there are deposits of sodium nitrate or Chile saltpeter, containing minor amounts of potassium nitrate. About two-thirds of the Chilean material normally goes to Europe and about one-fourth to the United States. The supply has been commercially controlled chiefly by Great Britain and by Chilean companies backed by British and German capital.

The dependence of the world on Chile became painfully apparent during the war. Germany was the only nation which had developed other sources of nitrogenous material to any great extent. The other nations were dependent in a very large degree on the mineral nitrates, both for fertilizer and munition purposes. Total demands far exceeded the total output from Chile, requiring international agreement as to the division of the output among the nations. The stream of several hundred ships carrying nitrates from Chile was one of the vital war arteries. This situation led to strenuous efforts in the belligerent countries toward the development of other sources of nitrogen. The United States, under governmental appropriation, began the building of extensive plants for the fixation of nitrogen from the air, and the building of by-product coke ovens in the place of the old wasteful beehive ovens was accelerated. Germany before the war had already gone far in both of these directions, not only within her own boundaries, but in the building of fixation plants in Scandinavia and Switzerland. War conditions required further development of these processes in Germany, with the result that this country was soon entirely self-supporting in this regard. One of the effects was the almost complete elimination in Germany of anything but the by-product process of coking coal.

War-time development of the nitrogen industry in the United States for munition purposes brought the domestic production almost up to the pre-war requirements for fertilizers alone. With the increasing demand for fertilizers and with the cheapness of the Chilean supply of natural nitrates, it is likely that the United States will continue for a good many years to import considerable amounts of Chilean nitrates. It may be noted that, although this country normally consumes about one-fourth of the Chilean product, American interests commercially control less than one-twentieth of the output. Presumably, if for no other purpose than future protection, effort will be made to develop the domestic industry to a point where in a crisis the United States could be independent of Chile. Particularly may an increase in the output of by-product ammonia from coke manufacture be looked for (see also pp. 118-119), since nitrogenous material thus produced need bear no fixed part of the cost of production, and requires no protective tariff.

The reserves of Chilean nitrate are known to be sufficient for world requirements for an indefinitely long future.

GEOLOGIC FEATURES

Mineral nitrates in general, and particularly those of soda and potash, are readily soluble at ordinary temperatures. Mineral nitrate deposits are therefore very rare, and are found only in arid regions or other places where they are protected from rain and ground-water. The only large deposits known are those of northern Chile and some extensions in adjacent parts of Peru and Bolivia. These are located on high desert plateaus, where there is almost a total absence of rain, and form blankets of one to six feet in thickness near the surface. The most important mineral, the sodium nitrate or Chile saltpeter, is mingled with various other soluble salts, including common salt, borax minerals, and potassium nitrate, and with loose clay, sand, and gravel. The nitrate deposits occur largely around and just above slight basin-like depressions in the desert which contain an abundance of common salt. The highest grade material contains 40 to 50 per cent of sodium nitrate, and material to be of shipping grade must run at least 12 to 15 per cent.

The origin of the nitrate beds is commonly believed to be similar to that of beds of rock salt (pp. 295-298), borax, and other saline residues. The source of the nitrogen was probably organic matter in the soil, such as former deposits of bird guano, bones (which are actually found in the same desert basin), and ancient vegetable matter. By the action of nitrifying bacteria on this organic matter, nitrate salts are believed to have formed which were leached out by surface and ground waters, and probably carried in solution to enclosed bodies of water. Here they became mingled with various other salts, and all were precipitated out as the waters of the basins evaporated. Deliquescence and later migration of the more soluble nitrates resulted in their accumulation around the edges of the basins. The nitrate beds are thus essentially a product of desiccation.

While the origin just set forth is rather generally accepted, several other theories have been advanced. It has been suggested that the deposits were not formed in water basins, but that ground water carrying nitrates in solution has been and is rising to the surface,—where, under the extremely arid conditions, it evaporates rapidly, leaving the nitrates mixed with the surface clays. One group of writers accounts for the deposits by the fixation of atmospheric nitrogen through electrical phenomena. Still others note the frequent presence of nitrogen in volcanic exhalations and the association of the Chilean nitrate beds with surface volcanic rocks; they suggest that these rocks were the source of the nitrogen, which under unusual climatic conditions was leached out and then deposited by evaporation.

PHOSPHATES

ECONOMIC FEATURES

The principal use of natural phosphates is in the manufacture of fertilizers. They are also used in the manufacture of phosphorus, phosphoric acid, and other phosphorus compounds, for matches, for certain metallurgical operations, and for gases used in military operations.

The material mined is mainly a phosphate of lime (tricalcium phosphate). To make it available for plant use, it is treated with sulphuric acid to form a soluble superphosphate; hence the importance of sulphuric acid, and its mineral sources pyrite and sulphur, in the fertilizer industry. A small percentage of the phosphate is also ground up and applied directly to the soil in the raw form. Other phosphatic materials are the basic slag from phosphatic iron ores made into Thomas-process steel, guano from the Pacific islands, and bone and refuse (tankage) from the cattle raising and packing countries. These materials are used for the same purposes as the natural phosphates.

The United States is the largest factor in the world's phosphate industry, with reference both to production and reserves.

The largest and most available of the European sources are in Tunis and Algeria, under French control, and in Egypt, under English control. Belgium and northern France have been considerable producers of phosphates, but, with the development of higher grade deposits in other countries, their production has fallen to a very small fraction of the world's total. There also has been very small and insignificant production in Spain and Great Britain. Russia has large reserves which are practically unmined.

While there is comparatively little phosphate rock in western Europe, a considerable amount of the phosphate supply is obtained as a by-product from Thomas slag, derived from phosphatic iron ores. These ores are chiefly from Lorraine and Sweden, but English and Russian ores can be similarly used.

Outside of Europe and the United States, there are smaller phosphate supplies in Canada, the Dutch West Indies, Venezuela, Chile, South Australia, New Zealand, and several islands of the Indian and South Pacific Oceans. None of these has yet contributed largely to world production, and their distance from the principal consuming countries bordering the North Atlantic basin is so great that there is not likely to be any great movement to this part of the world. On the other hand, some of the South Sea islands have large reserves of exceptionally high grade guano and bone phosphates, which will doubtless be used in increasing amounts for export to Japan, New Zealand, and other nearby countries. The most important of these islands are now controlled by Great Britain, Japan, and France.

A striking feature of the situation is that the central European countries, which have been large consumers of phosphate material, have lost not only the Pacific island phosphates but the Lorraine phosphatic iron ores, and are now almost completely dependent on British, French, and United States phosphate.

In the United States, reserves of phosphate are very large. They are mined principally in Florida, Tennessee, and South Carolina; but great reserves, though of lower grade, are known in Arkansas, Montana, Idaho, Wyoming, and Utah. There are possibilities for the development of local phosphate industries in the west, in connection with the manufacture of sulphuric acid from waste smelting gases at nearby mining centers. The Anaconda Copper Mining Company has taken up the manufacture of superphosphate as a means of using sulphuric acid made in relation to its smelting operations. The United States is independent in phosphate supplies and has a surplus for export. This country, England, and France exercise control of the greater part of the world's supply of phosphatic material. In competition for world trade, the Florida and Carolina phosphates are favorably situated for export, but there is strong competition in Europe from the immense fields in French North Africa, which are about equally well situated.

GEOLOGIC FEATURES

Small amounts of phosphorus are common in igneous rocks, in the form of the mineral apatite (calcium phosphate with calcium chloride or fluoride). Apatite is especially abundant in some pegmatites. In a few places, as in the Adirondacks where magnetic concentration of iron ores leaves a residue containing much apatite, and in Canada and Spain where veins of apatite have been mined, this material is used as a source of phosphate fertilizer. The great bulk of the world's phosphate, however, is obtained from other sources—sedimentary and residual beds described below.

Phosphorus in the rocks is dissolved in one form or another by the ground-waters; a part of it is taken up by land plants and animals for the building of their tissues, and another part goes in solution to the sea to be taken up by sea plants and animals. In places where the bones and excrements of land animals or the shells and droppings of sea animals accumulate, deposits of phosphatic material may be built up.

In certain places where great numbers of sea birds congregate, as on desert coasts and oceanic islands, guano deposits have been formed. Some of them, like the worked-out deposits of Peru and Chile, are in arid climates and have been well preserved. Others, like those of the West Indies and Oceania, are subjected to the action of occasional rains; and to a large extent the phosphates have been leached out, carried down, and reprecipitated, permeating and partially replacing the underlying limestones. In this way deposits have been formed containing as high as 85 per cent calcium phosphate.

Even more important bodies of phosphates have been produced by the accumulation of marine animal remains, probably with the aid of joint chemical, bacterial, and mechanical precipitation. These processes have formed the chief productive deposits of the world, including those of the United States, northern Africa, and Russia, and also the phosphatic iron ores of England and central Europe. The sedimentary features of many phosphate rocks, particularly their oolitic textures, show a marked similarity to the features of the Clinton type of iron ores (pp. 166-167).

The marine phosphate beds originally consist principally of calcium phosphate and calcium carbonate in varying proportions. Depending on the amount of secondary enrichment, they form two main types of deposits. The extensive beds of the western United States (in the upper Carboniferous) are hard, and very little enrichment by weathering has taken place; they carry in their richer portions 70 to 80 per cent calcium phosphate, and large sections range only from about 30 to 50 per cent. In the southeastern deposits (Silurian and Devonian in Tennessee and Tertiary in the Carolinas and Florida), there has been considerable enrichment, the rock is softer, and the general grade ranges from 65 to 80 per cent. Both calcium carbonate and calcium phosphate are soluble in ordinary ground waters, but the carbonate is the more soluble of the two. Thus the carbonate has been dissolved out more rapidly, and in addition descending waters carrying the phosphate have frequently deposited it to pick up the carbonate. These enriching processes, sometimes aided by mechanical concentration, have formed high-grade deposits both in the originally phosphatic beds and in various underlying strata. Concretionary and nodular textures are common. The "pebble" deposits of Florida consist of the phosphatic materials broken up and worked over by river waters and advancing shallow seas.

PYRITE

ECONOMIC FEATURES

The principal use of pyrite is in the manufacture of sulphuric acid. Large quantities of acid are used in the manufacture of fertilizers from phosphate rock, and during war times in the manufacture of munitions. Sulphuric acid converts the phosphate rock into superphosphate, which is soluble and available for plant use. Other uses of the acid are referred to in connection with sulphur. Pyrite is also used in Europe for the manufacture of paper from wood-pulp, but in the United States native sulphur has thus far been exclusively used for this purpose. The residue from the roasting of pyrite is a high-grade iron ore material frequently very low in phosphorus, which is desirable in making up mixtures for iron blast furnaces.

Most of the countries of Europe are producers of pyrite, and important amounts are also produced in the United States and Canada. The European production is marketed mainly on that continent, but considerable amounts come to the United States from Spain.

Before the war domestic sources supplied a fourth to a third of the domestic demand for pyrite. Imports came mainly from Spain and Portugal to consuming centers on the Atlantic seaboard. The curtailment of overseas imports of pyrite during the war increased domestic production by about a third and resulted also in drawing more heavily on Canadian supplies, but the total was not sufficient to meet the demand. The demand was met by the increased use of sulphur from domestic deposits (p. 109). At the close of the war supplies of pyrite had been accumulated to such an extent that, with the prospect of reopening of Spanish importation, pyrite production in the United States practically ceased. War experience has demonstrated the possibility of substitution of sulphur, which the United States has in large and cheaply mined quantities. The future of the pyrite industry in the United States therefore looks cloudy, except for supplies used locally, as in the territory tributary to the Great Lakes, and except for small amounts locally recovered as by-products in the mining of coal or from ores of zinc, lead, and copper. Pyrite production in the past has been chiefly in the Appalachian region, particularly in Virginia and New York, and in California.

GEOLOGIC FEATURES

Pyrite, the yellow iron sulphide, is the commonest and most abundant of the metallic sulphides. It is formed under a large variety of conditions and associations. Marcasite and pyrrhotite, other iron sulphide minerals, are frequently found with pyrite and are used for the same purposes.

The great deposits of Rio Tinto, Spain, which produce about half of the world's pyrite, were formed by replacement of slates by heated solutions from nearby igneous rocks. The ores are in lenticular bodies, and consist of almost massive pyrite with a small amount of quartz and scattered grains and threads of chalcopyrite (copper-iron sulphide). They carry about 50 per cent of sulphur, and the larger part carries about 2 per cent of copper which is also recovered.

Similar occurrences of pyrite on a smaller scale are known in many places. Pyrite is very commonly found in vein and replacement deposits of gold, silver, copper, lead, and zinc. In the Mississippi valley it is extracted as a by-product from the lead and zinc ores, and in the Cordilleran region large quantities of by-product pyrite could easily be produced if there were a local demand. The pyrite deposits of the Appalachian region are chiefly lenses in schists; they are of uncertain origin though some are believed to have been formed by replacement of metamorphosed limestones and schists.

Under weathering conditions pyrite oxidizes, the sulphur forming sulphuric acid,—an important agent in the secondary enrichment of copper and other sulphides,—and the iron forming the minerals hematite and limonite in the shape of a "gossan" or "iron-cap."

Pyrite is likewise frequently found in sediments, apparently being formed mainly by the reducing action of organic matter on iron salts in solution. In Illinois and adjacent states it is obtained as a by-product of coal mining.

SULPHUR

ECONOMIC FEATURES

Sulphur is used for many of the same purposes as pyrite. Under pre-war conditions, the largest use in the United States was in the manufacture of paper pulp by the sulphite process. Minor uses were in agriculture as a fungicide and insecticide, in vulcanizing rubber, and in the manufacture of gunpowder. About 5 per cent of the sulphur of the United States was used in the manufacture of sulphuric acid. During the war this use was greatly increased because of the shortage of pyrite and the large quantities of sulphuric acid necessary for the manufacture of explosives. The replacement of pyrite by sulphur in the manufacture of sulphuric acid has continued since the war, and in the future is likely to continue to play an important part. Sulphuric acid is an essential material for a great range of manufacturing processes. Some of its more important applications are: in the manufacture of superphosphate fertilizer from phosphate rock; in the refining of petroleum products; in the iron, steel, and coke industries; in the manufacture of nitroglycerin and other explosives; and in general metallurgical and chemical practice.

The United States is the world's largest sulphur producer. The principal foreign countries producing important amounts of sulphur are Italy, Japan, Spain, and Chile. Europe is the chief market for the Italian sulphur. In spite of increased demands in Europe the Italian production has decreased as the result of unfavorable labor, mining, and transportation conditions, and the deficit has had to be met from the United States. Japan's sulphur production has been increasing. Normally about half of the material exported comes to the United States to supply the needs of the paper industry in the Pacific states, and half goes to Australia and other British colonies. Spain's production is relatively small and has been increasing slowly; most of it is consumed locally. Chile's small production is mainly consumed at home and large additional amounts are imported.

The sulphur output of the United States, which in 1913-14 was second to Italy, now amounts to three-fourths of the entire output of the world, and the United States has become a large exporter of sulphur. Supplies are ample and production increasing, with the result that the United States can not only meet its own demands, but can use this commodity extensively in world trade. Small amounts of sulphur are mined in some of the western states, but over 98 per cent of the production comes from Louisiana and Texas.

GEOLOGIC FEATURES

Native sulphur is found principally in sedimentary beds, where it is associated with gypsum and usually with organic matter. Deposits of this type are known in many places, the most important being those of Sicily and of the Gulf Coast in the United States. In the latter region beds of limestone carry lenses of sulphur and gypsum which are apparently localized in dome-like upbowings of the strata. The deposits are overlain by several hundred feet of loose, water-bearing sands, through which it is difficult to sink a shaft. An ingenious and efficient process of mining is used whereby superheated water is pumped down to melt the sulphur, which is then forced to the surface by compressed air and allowed to consolidate in large bins. The Sicilian deposits are similar lenses in clayey limestones containing 20 to 25 per cent of sulphur, associated with gypsum and bituminous marl; they are mined by shafts.

Concerning the origin of these deposits several theories have been advanced. It has been thought that the materials for the deposits were precipitated at the same time as the enclosing sediments; and that the sulphur may have been formed by the oxidation of hydrogen sulphide in the precipitating waters through the agency of air or of sulphur-secreting bacteria, or that it may have been produced by the reduction of gypsum by organic matter or bacteria. Others have suggested that hot waters rising from igneous rocks may have brought in both the sulphur and the gypsum, which in crystallizing caused the upbowing of the strata which is seen in the Gulf fields (see also p. 298).

Native sulphur is also found in mineral springs from which hydrogen sulphide issues, where it is produced by the oxidation of the hydrogen sulphide. It likewise occurs in fissures of lava and around volcanic vents, where it has probably been formed by reactions between the volcanic gases and the air. The Japanese and Chilean deposits are of the volcanic type.

POTASH

ECONOMIC FEATURES

Potash is used principally as a component of fertilizers in agriculture. It is also used in the manufacture of soap, certain kinds of glass, matches, certain explosives, and chemical reagents.

For a long time potash production was essentially a German monopoly. The principal deposits are in the vicinity of Stassfurt in north central Germany (about the Harz Mountains). Stassfurt salts are undoubtedly ample to supply the world's needs of potash for an indefinite future. However, other deposits, discovered in the Rhine Valley in Alsace in 1904, have been proved to be of great extent; and though the production has hitherto been limited by restrictions imposed by the German Government, it has nevertheless become considerable.[15] The grade (18 per cent K_{2}O) is superior to the general run of material taken from the main German deposits, and the deposits have a regularity of structure and uniformity of material favorable to cheaper mining and refining than obtains in the Stassfurt deposits.

Other countries have also developed supplies of potash, some of which will probably continue to produce even in competition with the deposits of recognized importance referred to above. Noteworthy among the newer developments are those in Spain.[16] These have not yet produced on any large scale, but their future production may be considerable. Less important deposits are known in Galicia, Tunis, Russia, and eastern Abyssinia, and the nitrate deposits of Chile contain a small percentage of potash which is being recovered in some of the operations.

Prior to the war the United States obtained its potash from Germany. The German potash industry was well organized and protected by the German Government, which made every effort to maintain a world monopoly. During the war the potash exports from Germany were cut off, excepting exports to the neutrals immediately adjoining German territory. The result in the United States was that the price of potash rose so far as to greatly diminish its use as fertilizer.

The consequent efforts to increase potash production in the United States met with considerable success, but the maximum production attained was only about one-fourth of the ordinary pre-war requirements. The principal American sources are alkaline beds and brines in Nebraska, Utah, and California, and especially at Searles Lake, California. These furnished 75 per cent of the total output. Minor amounts have been extracted in Utah from the mineral alunite (a sulphate of potassium and aluminum), in Wyoming from leucite (a potassium-aluminum silicate), in California from kelp or seaweed, and in various localities from cement-mill and blast-furnace dusts, from wood ashes, from wool washings, from the waste residues of distilleries and beet-sugar refineries, and from miscellaneous industrial wastes. At the close of the war, sufficient progress had been made in the potash industry to indicate that the United States might become self-supporting in the future, though at high cost. The renewal of importation of cheap potash from Germany, with probable further offerings from Alsace and Spain, makes it impossible for the United States potash production to continue; except, perhaps, for the recovery of by-products which will go on in connection with other industries. Demand for a protective tariff has been the inevitable result (see Chapters XVII and XVIII).

GEOLOGIC FEATURES

Potassium is one of the eight most abundant elements in the earth. It occurs as a primary constituent of most igneous rocks, some of which carry percentages as high as those in commercial potash salts used for fertilizers. It is present in some sediments and likewise occurs in many schists and gneisses. Various potassium silicates—leucite, feldspar, sericite, and glauconite—and the potassium sulphate, alunite, have received attention and certain of them have been utilized to a small extent, but none of them are normally able to compete on the market. Potential supplies are thus practically unlimited in amount and distribution. Deposits from which the potash can be extracted at a reasonable cost, however, are known in only a few places, where they have been formed as saline sediments.

In the decomposition of rocks the potash, like the soda, is readily soluble, but in large part it is absorbed and held by clayey materials and is not carried off. Potash is therefore more sparingly present in river and ocean waters than is soda, and deposits of potash salts are much rarer than those of rock salt and other sodium compounds. The large deposits in the Permian beds of Stassfurt, as well as those in the Tertiary of Alsace and Spain, have been formed by the evaporation of very large quantities of salt water, presumably sea water. They consist of potassium salts, principally the chloride, mixed and intercrystallized with chlorides and sulphates of magnesium, sodium, and calcium. In the Stassfurt deposits the potassium-magnesium salts occupy a relatively thin horizon at the top of about 500 feet of rock salt beds, the whole underlying an area about 200 miles long and 140 miles wide. The principal minerals in the potash horizon are carnallite (hydrous potassium-magnesium chloride), kieserite (hydrous magnesium sulphate), sylvite (potassium chloride), kainite (a hydrous double salt of potassium chloride and magnesium sulphate), and common salt (sodium chloride). The potash beds represent the last stage in the evaporation of the waters of a great closed basin, and the peculiar climatic and topographic conditions which caused their formation have been the subject of much speculation. This subject is further treated in the discussion of common salt beds (pp. 295-298).

In the United States the deposits at Searles Lake, California, have been produced by the same processes on a smaller scale. In this case evaporation has not been carried to completion, but the crystallization and separation out of other salts has concentrated the potassium (with the magnesium) in the residual brine or "mother liquor." The deposits of this lake or marsh also contain borax (see p. 276), and differ in proportions of salts from the Stassfurt deposits. This is due to the fact that they were probably derived, not from ocean waters, but from the leaching of materials from the rocks of surrounding uplands, transportation of these materials in solution by rivers and ground waters, and concentration in the desert basin by evaporation.

The alkali lakes of Nebraska are believed to be of very recent geologic origin. They lie in depressions in a former sand dune area, and contain large quantities of potash supposedly accumulated by leaching of the ashes resulting from repeated burnings of the grass in the adjacent country.

Of other natural mineral sources, alunite is the most important. The principal deposits worked are at Marysville, Utah, but the mineral is a rather common one in the western part of the United States, associated with gold deposits, as at Goldfield, Nevada. Alunite occurs as veins and replacement deposits, often in igneous associations, and is supposed to be of igneous source. Its origin is referred to in connection with the Goldfield ores (p. 230).

FOOTNOTES:

[15] Gale, Hoyt S., The potash deposits of Alsace: Bull. 715-B, U. S. Geol. Survey, 1920, pp. 17-55.

[16] Gale, Hoyt S., Potash deposits in Spain: Bull. 715-A, U. S. Geol. Survey, 1920, pp. 1-16.



CHAPTER VIII

THE ENERGY RESOURCES—COAL, OIL, GAS (AND ASPHALT)

COAL

ECONOMIC FEATURES

Coal overshadows all other mineral resources, except water, in production, value, and demand. It is the greatest of the energy sources—coal, petroleum, gas, and water power. Roughly two-thirds of the world's coal is used for power, one-sixth for smelting and metallurgical industries, and one-sixth for heating purposes. Coal constitutes over one-third of the railroad tonnage of the United States and is the largest single tonnage factor in international trade; 70 per cent of the pre-war tonnage of outgoing cargoes from England was coal.

World production and trade. The great coal-producing countries of the world border the North Atlantic basin. The United States produces about 40 per cent of the world's total, Great Britain about 20 per cent, and Germany about 20 per cent. Other countries producing coal stand in about the following order: Austria-Hungary, France, Russia, Belgium, Japan, China, India, Canada, and New South Wales. There is similarity in the major features of the distribution of coal production and of iron ore production. The great centers of coal production—the Pennsylvania and Illinois fields of the United States, the Midlands district of England, and the lower Rhine or Westphalian fields of Germany—are also the great centers of the iron and steel industries of these countries. As in the case of iron ore, there is rather a striking absence of important coal production in the southern hemisphere and in Asia. A significant item in the world's distribution of coal supplies is England's world-wide system of coaling stations for shipping.