The damage which earthquakes do to buildings is in most cases due to the fact that they sway their walls out of plumb, so that they are no longer in position to support the weight which they have to bear. The amount of this swaying is naturally very much greater than that which the earth itself experiences in the movement. A building of any height with its walls unsupported by neighbouring structures may find its roof rocked to and fro through an arc which has a length of feet, while its base moves only through a length of inches. The reader may see an example of this nature if he will poise a thin book or a bit of plank a foot long on top of a small table; then jarring the table so that it swings through a distance of say a quarter of an inch, he will see that the columnar object swings at its top through a much greater distance, and is pretty sure to be overturned.

Where a building carries a load in its upper parts, such as may be afforded by its heavy roof or the stores which it contains, the effect of an earthquake shock such as carries the earth to and fro becomes much more destructive than it might otherwise be. This weight lags behind when the earth slips forward in the first movement of the oscillation, with the effect that the walls of the building are pretty sure to be thrust so far beyond the perpendicular that they give way and are carried down by the weight which they bore. It has often been remarked in earthquake shocks that tall columns, even where composed of many blocks, survive a shock which overturns lower buildings where thin walls support several floors, on each of which is accumulated a considerable amount of weight. In the case of the column, the strains are even, and the whole structure may rock to and fro without toppling over. As the energy of the undulations diminish, it gradually regains the quiet state without damage. In the ordinary edifice the irregular disposition of the weight does not permit the uniform movement which may insure safety. Thus, if the city of Washington should ever be violently shaken, the great obelisk, notwithstanding that it is five hundred feet high, may survive a disturbance which would wreck the lower and more massive edifices which lie about it.

Where, as is fortunately rarely the case, the great shock comes to the earth in a vertical direction, the effect upon all movable objects is in the highest measure disastrous. In such a case buildings are crushed as if by the stroke of a giant's hand. The roofs and floors are at one stroke thrown to the foundations, and all the parts of the walls which are not supported by strong masonry continuous from top to bottom are broken to pieces. In such cases it has been remarked that the bodies of men are often thrown considerable distances. It is asserted, indeed, that in the Riobamba shock they were cast upward to the height of more than ninety feet. It is related that the solo survivor of a congregation which had hastened at the outset of the disturbance into a church was thrown by the greatest and most destructive shock upward and through a window the base of which was at the height of more than twenty feet from the ground.

It is readily understood that an earthquake shock may enter a building in any direction between the vertical and the horizontal. As the movement exhausts itself in passing from the place of its origin, the horizontal shocks are usually of least energy. Those which are accurately vertical are only experienced where the edifices are placed immediately over the point where the motion originates. It follows, therefore, that the destructive work of earthquakes is mainly performed in that part of the field where the motion is, as regards its direction, between the vertical and the horizontal—a position in which the edifice is likely to receive at once the destructive effect arising from the sharp upward thrust of the vertical movement and the oscillating action of that which is in a horizontal direction. Against strains of this description, where the movements have an amplitude of more than a few inches, no ordinary masonry edifice can be made perfectly safe; the only tolerable security is attained where the building is of well-framed timber, which by its elasticity permits a good deal of motion without destructive consequences. Even such buildings, however, those of the strongest type, may be ruined by the greater earthquakes. Thus, in the Mississippi Valley earthquake of 1811, the log huts of the frontiersmen, which are about as strong as any buildings can be made, were shaken to pieces by the sharp and reiterated shocks.

It is by no means surprising to find that the style of architecture adopted in earthquake countries differs from that which is developed in regions where the earth is firm-set. The people generally learn that where their buildings must meet the trials of earthquakes they have to be low and strong, framed in the manner of fortifications, to withstand the assault of this enemy. We observe that Gothic architecture, where a great weight of masonry is carried upon slender columns and walls divided by tall windows, though it became the dominant style in the relatively stable lands of northern Europe, never gained a firm foothold in those regions about the Mediterranean which are frequently visited by severe convulsions of the earth. There the Grecian or the Romanesque styles, which are of a much more massive type, retain their places and are the fashions to the present day. Even this manner of building, though affording a certain security against slight tremblings, is not safe in the greater shocks. Again and again large areas in southern Italy have been almost swept of their buildings by the destructive movements which occur in that realm. The only people who have systematically adapted their architectural methods to earthquake strains are the Japanese, who in certain districts where such risks are to be encountered construct their dwellings of wood, and place them upon rollers, so that they may readily move to and fro as the shock passes beneath them. In a measure the people of San Francisco have also provided against this danger by avoiding dangerous weights in the upper parts of their buildings, as well as the excessive height to which these structures are lifted in some of our American towns.

Earthquakes of sensible energy appear to be limited to particular parts of the earth's crust. The regions, indeed, where within the period of human history shocks of devastating energy have occurred do not include more than one fifteenth part of the earth's surface. There is a common notion that these movements are most apt to happen in volcanic regions. It is, indeed, true that sensible shocks commonly attend the explosions from great craters, but the records clearly show that these movements are very rarely of destructive energy. Thus in the regions about the base of Vesuvius and of AEtna, the two volcanoes of which most is known, the shocks have never been productive of extensive disaster. In fact, the reiterated slight jarrings which attend volcanic action appear to prevent the formation of those great and slowly accumulated strains which in their discharge produce the most violent tremblings of the earth. The greatest and most continuous earthquake disturbances of history—that before noted in the early days of this century, in the Mississippi Valley, where shocks of considerable violence continued for two years—came about in a field very far removed from active volcanoes. So, too, the disturbances beneath the Atlantic floor which originated the shocks that led to the destruction of Lisbon, and many other similar though less violent movements, are developed in a field apparently remote from living volcanoes. Eastern New England, which has been the seat of several considerable earthquakes, is about as far away from active vents as any place on the habitable globe. We may therefore conclude that, while volcanoes necessarily produce shocks resulting from the discharge of their gases and the intrusion of lava into the dikes which are formed about them, the greater part of the important shocks are in no wise connected with volcanic explosions.

With the exception of the earthquake in the Mississippi Valley, all the great shocks of which we have a record have occurred in or near regions where the rocks have been extensively disturbed by mountain-building forces, and where the indications lead us to believe that dislocations of strata, such as are competent to rive the beds asunder, may still be in progress. This, taken in connection with the fact that many of these shocks are attended by the formation of fault planes, which appear on the surface, lead us to the conclusion that earthquakes of the stronger kind are generally formed by the riving of fissures, which may or may not be developed upward to the surface. This view is supported by many careful observations on the effect which certain great earthquakes have exercised on the buildings which they have ravaged. The distinguished observer, Mr. Charles Mallet, who visited the seat of the earthquake which, in 1854, occurred in the province of Calabria in Italy, with great labour and skill determined the direction in which the shock moved through some hundreds of edifices on which it left the marks of its passage. Platting these lines of motion, he found that they were all referred to a vertical plane lying at the depth of some miles beneath the surface, and extending for a great distance in a north and south direction. This method of inquiry has been applied to other fields, with the result that in the case of all the instances which have been subjected to this inquiry the seat of the shock has been traced to such a plane, which can best be accounted for by the supposition of a fault.

The method pursued by Mr. Mallet in his studies of the origin of earthquakes, and by those who have continued his inquiry, may be briefly indicated as follows: Examining disrupted buildings, it is easy to determine those which have been wrecked by a shock that emerged from the earth in a vertical direction. In these cases, though tall walls may remain standing, the roofs and floors are thrown into the cellars. With a dozen such instances the plane of what is called the seismic vertical is established (seismos is the Greek for earthquake). Then on either side of this plane, which indicates the line but not the depth of the disturbance, other observations may be made which give the clew to the depth. Thus a building may be found where the northwest corner at its upper part has been thrown off. Such a rupture was clearly caused by an upward but oblique movement, which in the first half of the oscillation heaved the structure upwardly into the northwest, and then in the second half, or rebound, drew the mass of the building away from the unsupported corner, allowing that part of the masonry to fly off and fall to the ground. Constructing a line at right angles to the plane of the fracture, it will be found to intersect the plane, the position of which has been in part determined by finding the line where it intersects the earth, or the seismic vertical before noted. Multiplying such observations on either side of the last-mentioned line, the attitude of the underground parts of the plane, as well as the depth to which it attained, can be approximately determined.

It is worth while to consider the extent to which earthquake shocks may affect the general quality of the people who dwell in countries where these disturbances occur with such frequency and violence as to influence their lives. There can be no question that wherever earthquakes occur in such a measure as to produce widespread terror, where, recurring from time to time, they develop in men a sense of abiding insecurity, they become potent agents of degradation. All the best which men do in creating a civilization rests upon a sense of confidence that their efforts may be accumulated from year to year, and that even after death the work of each man may remain as a heritage to his kind. It is likely, indeed, that in certain realms, as in southern Italy, a part of the failure of the people to advance in culture is due to their long experience of such calamities, and the natural expectation that they will from time to time recur. In a similar way the Spanish settlements in Central and South America, which lie mostly in lands that are subject to disastrous shocks, may have been retarded by the despair, as well as the loss of property and life, which these accidents have so frequently inflicted upon them. It will not do, however, to attribute too much to such terrestrial influences. By far the most important element in determining the destiny of a people is to be found in their native quality, that which they owe to their ancestors of distant generations. In this connection it is well to consider the history of the Icelandic people, where a small folk has for a thousand years been exposed to a range and severity of trials, such as earthquakes, volcanic explosions, and dearth of harvests may produce, and all these in a measure that few if any other countries experience. Notwithstanding these misfortunes, the Icelanders have developed and maintained a civilization which in all else, except its material results, on the average transcends that which has been won by any other folk in modern times. If a people have the determining spirit which leads to high living, they can successfully face calamities far greater than those which earthquakes inflict.

It was long supposed that the regions where earthquakes are not noticeable by the unaided senses were exempt from all such disturbances. The observations which seismologists have made in recent years point to the conclusion that no part of the earth's surface is quite exempt from movements which, though not readily perceived, can be made visible by the use of appropriate instruments. With an apparatus known as the horizontal pendulum it is possible to observe vibrations which do not exceed in amplitude the hundredth part of an inch. This mechanism consists essentially of a slender bar supported near one end by two wires, one from above, the other from below. It may readily be conceived that any measurable movement will cause the longer end of the rod to sway through a considerable arc. Wherever such a pendulum has been carefully observed in any district, it has been found that it indicates the occurrence of slight tremors. Even certain changes of the barometer, which alter the weight of the atmosphere that rests upon the earth to the amount indicated by an inch in the height of the mercury column, appears in all cases to create such tremors. Many of these slight shocks may be due to the effect of more violent quakings, which have run perhaps for thousands of miles from their point of origin, and have thus been reduced in the amplitude of their movement. Others are probably due to the slight motion brought about through the chemical changes of the rocks, which are continuously going on. The ease with which even small motions are carried to a great distance may be judged by the fact that when the ground is frozen the horizontal pendulum will indicate the jarring due to a railway train at the distance of a mile or more from the track.

In connection with the earth jarring, it would be well to note the occurrence of another, though physically different, kind of movement, which we may term earth swayings, or massive movements, which slowly dislocate the vertical, and doubtless also the horizontal, position of points upon its surface. It has more than once been remarked that in mountain countries, where accurate sights have been taken, the heights of points between the extremities of a long line appear somewhat to vary in the course of a term of years. Thus at a place in the Apennines, where two buildings separated by some miles of distance are commonly intervisible over the crest of a neighbouring peak, it has happened that a change of level of some one of the points has made it impossible to see the one edifice from the other. Knowing as we do that the line of the seacoast is ever-changing, uprising taking place at some points and down-sinking at others, it seems not unlikely that these irregular swayings are of very common occurrence. Moreover, astronomers are beginning to remark the fact that their observatories appear not to remain permanently in the same position—that is, they do not have exactly the same latitude and longitude. Certain of these changes have recently been explained by the discovery of a new and hitherto unnoted movement of the polar axis. It is not improbable, however, that the irregular swaying of the earth's crust, due to the folding of strata and to the alterations in the volume of rocks which are continually going on, may have some share in bringing about these dislocations.

Measured by the destruction which was wrought to the interests of man, earthquakes deserve to be reckoned among the direst calamities of Nature. Since the dawn of history the records show us that the destruction of life which is to be attributed to them is to be counted by the millions. A catalogue of the loss of life in the accidents of this description which have occurred during the Christian era has led the writer to suppose that probably over two million persons have perished from these shocks in the last nineteen centuries. Nevertheless, as compared with other agents of destruction, such as preventable disease, war, or famine, the loss which has been inflicted by earth movements is really trifling, and almost all of it is due to an obstinate carelessness in the construction of buildings without reference to the risks which are known to exist in earthquake-ridden countries.

Although all our exact knowledge concerning the distribution of earthquakes is limited to the imperfect records of two or three thousand years, it is commonly possible to measure in a general way the liability to such accidents which may exist in any country by a careful study of the details of its topography. In almost every large area the process of erosion naturally leaves quantities of rock, either in the form of detached columns or as detrital accumulations deposited on steep slopes. These features are of relatively slow formation, and it is often possible to determine that they have been in their positions for a time which is to be measured by thousands of years. Thus, on inspecting a country such as North America, where the historic records cover but a brief time, we may on inquiry determine which portions of its area have long been exempt from powerful shocks. Where natural obelisks and steep taluses abound—features which would have disappeared if the region had been moved by great shocks—we may be sure that the field under inspection has for a great period been exempt from powerful shaking. Judged by this standard, we may safely say that the region occupied by the Appalachian Mountains has been exempt from serious trouble. So, too, the section of the Cordilleras lying to the east of what is commonly called the Great Basin, between the Rocky Mountains and the Sierra Nevada, has also enjoyed a long reign of peace. In glaciated countries the record is naturally less clear than in those parts of the world which have been subjected to long-continued, slow decay of the rocks. Nevertheless, in those fields boulders are often found poised in position which they could not have maintained if subjected to violent shaking. Judged by this evidence, we may say that a large part of the northern section of this continent, particularly the area about the Great Lakes, has been exempt from considerable shocks since the glacier passed away.

The shores which are subject to the visitations of the great marine waves, caused by earthquake shocks occurring beneath the bottom of the neighbouring ocean, are so swept by those violent inundations that they lose many features which are often found along coasts that have been exempted from such visitations. Thus wherever we find extensive and delicately moulded dunes, poised stones, or slender pinnacled rocks along a coast, we may be sure that since these features were formed the district has not been swept by these great waves.



Around the northern Atlantic we almost everywhere find the glacial waste here and there accumulated near the margin of the sea in the complicated sculptured outlines which are assumed by kame sands and gravels. From a study of these features just above the level of high tide, the writer has become convinced that the North Atlantic district has long been exempt from the assaults of other waves than those which are produced during heavy storms. At the present time the waves formed by earthquakes appear to be of destructive violence only on the west coast of South America, where they roll in from a region of the Pacific lying to the south of the equator and a few hundred miles from the shore of the continent, which appears to be the seat of exceedingly violent shocks. A similar field occurs in the Atlantic between the Lesser Antilles and the Spanish peninsula, but no great waves have come thence since the time of the Lisbon earthquake. The basin of the Caribbean and the region about Java appear to be also fields where these disturbances may be expected, though in each but one wave of this nature has been recorded. Therefore we may regard these secondary results of a submarine earthquake as seldom phenomena.

DURATION OF GEOLOGICAL TIME.

Although it is beyond the power of man to conceive any such lapses of time as have taken place in the history of this earth, it is interesting, and in certain ways profitable, to determine as near as possible in the measure of years the duration of the events which are recorded in the rocks. Some astronomers, basing their conclusions on the heat-containing power of matter, and on the rate at which energy in this form flows from the sun, have come to the conclusion that our planet could not have been in independent existence for more than about twenty million years. The geologist, however, resting his conclusions on the records which are the subject of his inquiry, comes on many different lines to an opinion which traverses that entertained by some distinguished astronomers. The ways in which the student of the earth arrives at this opinion will now be set forth.

By noting the amount of sediment carried forth to the sea by the rivers, the geologist finds that the lands of the earth—those, at least, which are protected by their natural envelopes of vegetation—are wearing down at a rate which pretty certainly does not exceed one foot in about five thousand years, or two hundred feet in a million years. Discovering at many places on the earth's surface deposits which originally had a thickness of five thousand feet or more, which have been worn down to the depths of thousands of feet in a single rather brief section of geological time, the student readily finds himself prepared to claim that a period of from five to ten million years has often been required for the accomplishment of but a very small part of the changes which he knows to have occurred on this earth.

As the geologist follows down through the sections of the stratified rocks, and from the remains of strata determines the erosion which has borne away the greater part of the thick deposits which have been exposed to erosion, he comes upon one of those breaks in the succession, or encounters what is called an unconformity, as when horizontal strata lie against those which are tilted. In many cases he may observe that at this time there was a great interval unrepresented by deposits at the place where his observations are made, yet a great lapse of time is indicated by the fact that a large amount of erosion took place in the interval between the two sets of beds.

Putting together the bits of record, and assuming that the rate of erosion accomplished by the agents which operate on the land has always been about the same, the geologist comes to the conclusion that the section of the rocks from the present day to the lowest strata of the Laurentian represents in the time required for their formation not less than a hundred million years; more likely twice that duration. To this argument objection is made by some naturalists that the agents of erosion may have been more active in the past than they are at present. They suggest that the rainfall may have been much greater or the tides higher than they now are. Granting all that can be claimed on this score, we note the fact that the rate of erosion evidently does not increase in anything like a proportionate way with the amount of rainfall. Where a country is protected by its natural coating of vegetation, the rain is delivered to the streams without making any considerable assault upon the surface of the earth, however large the fall may be. Moreover, the tides have little direct cutting power; they can only remove detritus which other agents have brought into a condition to be borne away. The direct cutting power of the tidal movement does not seem to be much greater in the Bay of Fundy, where the maximum height of the waves amounts to fifty feet, than on the southern coast of Massachusetts, where the range is not more than five. So far as the observer can judge, the climatal conditions and the other influences which affect the wear of rocks have not greatly varied in the past from what they are at the present day. Now and then there have been periods of excessive erosion; again, ages in which large fields were in the conditions of exceeding drought. It is, however, a fair presumption that these periods in a way balance each other, and that the average state was much like that which we find at present.

If after studying the erosive phenomena exhibited in the structure of the earth the student takes up the study of the accumulations of strata, and endeavours to determine the time required for the laying down of the sediments, he finds similar evidence of the earth's great antiquity. Although the process of deposition, which has given us the rocks visible in the land masses, has been very much interrupted, the section which is made by grouping the observations made in various fields shows that something like a maximum thickness of a hundred and fifty thousand feet of beds has been accumulated in that part of geologic time during which strata were being laid down in the fields that are subjected to our study. Although in these rocks there are many sets of beds which were rapidly formed, the greater part of them have been accumulated with exceeding slowness. Many fine shales, such as those which plentifully occur in the Devonian beds of this country, must have required a thousand years or more for the deposition of the materials that now occupy an inch in depth. In those sections a single foot of the rock may well represent a period of ten thousand years. In many of the limestones the rate of accumulation could hardly have been more speedy. The reckoning has to be rough, but the impression which such studies make upon the mind of the unprejudiced observer is to the effect that the thirty miles or so of sedimentary deposits could not have been formed in less than a hundred million years. In this reckoning it should be noted that no account is taken of those great intervals of unrecorded time, such as elapsed between the close of the Laurentian and the beginning of the Cambrian periods.

There is a third way in which we may seek an interpretation of duration from the rocks. In each successive stage of the earth's history, in different measure in the various ages, mountains were formed which in time, during their exposure to the conditions of the land, were worn down to their roots and covered by deposits accumulated during the succeeding ages. A score or more of these successively constructed series of elevations may readily be observed. Of old, it was believed that mountain ranges were suddenly formed, but there is, however, ample evidence to prove that these disturbed portions of the strata were very gradually dislocated, the rate of the mountainous growth having been, in general, no greater in the past than it is at the present day, when, as we know full well, the movements are going on so slowly that they escape observation. Only here and there, as an attendant on earthquake shocks or other related movements of the crust, do we find any trace of the upward march which produces these elevations. Although not a subject for exact measurements, these features of mountain growth indicate a vast lapse of time, during which the elevations were formed and worn away.

Yet another and very different method by which we may obtain some gauge of the depths of the past is to be found in the steps which have led organic life from its lowest and earliest known forms to the present state of advancement. Taking the changes of species which have occurred since the beginning of the last ice epoch, we find that the changes which have been made in the organic life have been very small; no naturalist who has obtained a clear idea of the facts will question the statement that they are not a thousandth part of the alterations which have occurred since the Laurentian time. The writer is of the opinion that they do not represent the ten thousandth part of those vast changes. These changes are limited in the main to the disappearance of a few forms, and to slight modifications in those previously in existence which have survived to the present day. So far as we can judge, no considerable step in the organic series has taken place in this last great period of the earth's history, although it has been a period when, as before noted, all the conditions have combined to induce rapid modifications in both animals and plants. If, then, we can determine the duration of this period, we may obtain a gauge of some general value.

Although we can not measure in any accurate way the duration of the events which have taken place since the last Glacial period began to wane, a study of the facts seems to show that less than a hundred thousand years can not well be assumed for this interval. Some of the students who have approached the subject are disposed to allow a period of at least twice this length as necessary for the perspective which the train of events exhibits. Reckoning on the lowest estimate, and counting the organic changes which take place during the age as amounting to the thousandth part of the organic changes since the Laurentian age, we find ourselves in face once again of that inconceivable sum which was indicated by the physical record.

Here, again, the critics assert that there may have been periods in the history of the earth when the changes of organic life occurred in a far swifter manner than in this last section of the earth's history. This supposition is inadmissible, for it rests on no kind of proof; it is, moreover, contraindicated by the evident fact that the advance in the organic series has been more rapid in recent time than at any stage of the past. In a word, all the facts with which the geologist deals are decidedly against the assumption that terrestrial changes in the organic or the inorganic world ever proceed in a spasmodic manner. Here and there, and from time to time, local revolutions of a violent nature undoubtedly occur, but, so far as we may judge from the aspect of the present or the records of the past, these accidents are strictly local; the earth has gone forward in its changes much as it is now advancing. Its revolutions have been those of order rather than those of accident.

The first duty of the naturalist is to take Nature as he finds it. He must avoid supposing any methods of action which are not clearly indicated in the facts that he observes. The history of his own and of all other sciences clearly shows that danger is always incurred where suppositions as to peculiar methods of action are introduced into the interpretation. It required many centuries of labour before the students of the earth came to adopt the principle of explaining the problems with which they had to deal by the evidence that the earth submitted to them. Wherever they trusted to their imaginations for guidance, they fell into error. Those who endeavour to abbreviate our conception of geologic time by supposing that in the olden days the order of events was other than that we now behold are going counter to the best traditions of the science.

Although the aspect of the record of life since the beginning of the Cambrian time indicates a period of at least a hundred million years, it must not be supposed that this is the limit of the time required for the development of the organic series. All the important types of animals were already in existence in that ancient period with the exception of the vertebrates, the remains of which have apparently now been traced down to near the Cambrian level. In other words, at the stage where we first find evidence of living beings the series to which they belong had already climbed very far above the level of lifeless matter. Few naturalists will question the statement that half the work of organic advance had been accomplished at the beginning of the Cambrian rocks. The writer is of the opinion that the development which took place before that age must have required a much longer period than has elapsed from that epoch to the present day. We thus come to the conclusion that the measurement of duration afforded by organic life indicates a yet more lengthened claim of events, and demands more time than appears to be required for the formation of the stratified rocks.

The index of duration afforded by the organic series is probably more trustworthy than that which is found in the sedimentary strata, and this for the reason that the records of those strata have been subjected to numerous and immeasurable breaks, while the development of organic life has of necessity been perfectly continuous. The one record can at any point be broken without interrupting the sequences; the other does not admit of any breaches in the continuity.

THE MOON.

Set over against the earth—related to, yet contrasted with it in many ways—the moon offers a most profitable object to the student of geology. He should often turn to it for those lessons which will be briefly noted.

In the beginning of their mutual history the materials of earth and moon doubtless formed one vaporous body which had been parted from the concentrating mass of the sun in the manner noted in the sketch of the history of the solar system. After the earth-moon body had gathered into a nebulous sphere, it is most likely that a ring resembling that still existing about Saturn was formed about the earth, which in time consolidated into the satellite. Thenceforth the two bodies were parted, except for the gravitative attraction which impelled them to revolve about their common centre of gravity, and except for the light and heat they might exchange with one another.

The first stages after the parting of the spheres of earth and moon appear to have been essentially the same in each body. Concentrating upon their centres, they became in time fluid by heat; further on, they entered the rigid state—in a word, they froze—at least in their outer parts. At this point in their existence their histories utterly diverge; or rather, we may say, the development of the earth continued in a vast unfolding, while that of the moon appears to have been absolutely arrested in ways which we will now describe.

With the naked eye we see on the moon a considerable variation in the light of different parts of its surface; we discern that the darker patches appear to be rudely circular, and that they run together on their margins. Seeing this little, the ancients fancied that our satellite had seas and lands like the earth. The first telescopes did not dispel their fancies; even down to the early part of this century there were astronomers who believed the moon to be habitable; indeed, they thought to find evidence that it was the dwelling place of intelligent beings who built cities, and who tried to signal their intellectual kindred of this planet. When, however, strong glasses were applied to the exploration, these pleasing fancies were rudely dispelled.

Seen with a telescope of the better sort, the moon reveals itself to be in large part made up of circular depressions, each surrounded by a ringlike wall, with nearly level but rough places between. The largest of these walled areas is some four hundred miles in diameter; thence they grade down to the smallest pits which the glass can disclose, which are probably not over as many feet across. The writer, from a careful study of these pits, has come to the conclusion that the wider are the older and the smaller the last formed. The rude elevations about these pits—some of which rise to the height of ten thousand feet or more—constitute the principal topographic reliefs of the lunar surface. Besides the pits above mentioned, there are numerous fractures in the surface of the plains and ringlike ridges; on the most of these the walls have separated, forming trenches not unlike what we find in the case of some terrestrial breaks such as have been noted about volcanoes and elsewhere. It may be that the so-called canals of Mars are of the same nature.



The most curious feature on the moon's surface are the bands of lighter colour, which, radiating from certain of the volcanolike pits—those of lesser size and probably of latest origin—extend in some cases for five hundred miles or more across the surface. These light bands have never been adequately explained. It seems most likely that they are stains along the sides of cracks, such as are sometimes observed about volcanoes.

The eminent peculiarity of the moon is that it is destitute of any kind of gaseous or aqueous envelope. That there is no distinct atmosphere is clearly shown by the perfectly sharp and sudden way in which the light of a star disappears when it goes behind the moon and the clear lines of the edge of the satellite in a solar eclipse. The same evidence shows that there is no vapour of water; moreover, a careful search which the writer has made shows that the surface has none of those continuous down grades which mark the work of water flowing over the land. Nearly all of the surface consists of shallow or deep pits, such as could not have been formed by water action. We therefore have not only to conclude that the moon is waterless, but that it has been in this condition ever since the part that is turned toward us was shaped.

As the moon, except for the slight movement termed its "libration," always turns the same face to us, so that we see in all only about four sevenths of its surface, it has naturally been conjectured that the unseen side, which is probably some miles lower than that turned toward us, might have a different character from that which we behold. There are reasons why this is improbable. In the first place, we see on the extreme border of the moon, when the libration turns one side the farthest around toward the earth, the edge of a number of the great walled pits such as are so plenty on the visible area; it is fair to assume that these rings are completed in the invisible realm. On this basis we can partly map about a third of the hidden side. Furthermore, there are certain bands of light which, though appearing on the visible side, evidently converge to some points on the other. It is reasonable to suppose that, as all other bands radiate from walled pits, these also start from such topographic features. In this way certain likenesses of the hidden area to that which is visible is established, thus making it probable that the whole surface of the satellite has the same character.

Clearly as the greater part of the moon is revealed to us—so clearly, indeed, that it is possible to map any elevation of its surface that attains the height of five hundred feet—the interpretation of its features in the light of geology is a matter of very great difficulty. The main points seem to be tolerably clear; they are as follows: The surface of the moon as we see it is that which was formed when that body, passing from the state of fluidity from heat, formed a solid crust. The pits which we observe on its surface are the depressions which were formed as the mass gradually ceased to boil. The later formed of these openings are the smaller, as would be the case in such a slowing down of a boiling process.

As the diameter of the moon is only about one fourth of that of the earth, its bulk is only about one sixteenth of that of its planet; consequently, it must have cooled to the point of solidification ages before the larger sphere attained that state. It is probable that the same changeless face that we see looked down for millions of years on an earth which was still a seething, fiery mass. In a word, all that vast history which is traceable in the rocks beneath our feet—which is in progress in the seas and lands and is to endure for an inconceivable time to come—has been denied our satellite, for the reason that it had no air with which to entrap the solar heat and no water to apply the solar energy to evolutionary processes. The heat which comes upon the moon as large a share for each equal area as it comes upon the earth flies at once away from the airless surface, at most giving it a temporary warmth, but instituting no geological work unless it be a little movement from the expansion and contraction of the rocks. During the ages in which the moon has remained thus lifeless the earth, owing to its air and water, has applied a vast amount of solar energy to geological work in the development and redevelopment of its geological features and to the processes of organic life. We thus see the fundamental importance of the volatile envelopes of our sphere, how absolutely they have determined its history.

It would be interesting to consider the causes which led to the absence of air and water on the moon, but this matter is one of the most debatable of all that relates to that sphere; we shall therefore have to content ourselves with the above brief statements as to the vast and far-acting effects which have arisen from the non-existence of those envelopes on our nearest neighbour of the heavens.

METHODS IN STUDYING GEOLOGY.

So far as possible the preceding pages, by the method adopted in the presentation of facts, will serve to show the student the ways in which he may best undertake to trace the order of events exhibited in the phenomena of the earth. Following the plan pursued, we shall now consider certain special points which need to be noted by those who would adopt the methods of the geologist.

At the outset of his studies it may be well for the inquirer to note the fact that familiarity with the world about him leads the man in all cases to a certain neglect and contempt of all the familiar presentations of Nature. We inevitably forget that those points of light in the firmament are vast suns, and we overlook the fact that the soil beneath our feet is not mere dirt, but a marvellous structure, more complicated in its processes than the chemist's laboratory, from which the sustenance of our own and all other lives is drawn. We feel our own bodies as dear but commonplace possessions, though we should understand them as inheritances from the inconceivable past, which have come to us through tens of thousands of different species and hundreds of millions of individual ancestors. We must overlook these things in our common life. If we could take them into account, each soul would carry the universe as an intellectual burden.

It is, however, well from time to time to contemplate the truth, and to force ourselves to see that all this apparently simple and ordinary medley of the world about us is a part of a vast procession of events, coming forth from the darkness of the past and moving on beyond the light of the present day. Even in his professional work the naturalist of necessity falls into the commonplace way of regarding the facts with which he deals. If he be an astronomer, he catalogues the stars with little more sense of the immensities than the man who keeps a shop takes account of his wares. Nevertheless, the real profit of all learning is in the largeness of the understanding which it develops in man. The periods of growth in knowledge are those in which the mind, enriched by its store, enlarges its conception while it escapes from commonplace ways of thought. With this brief mention of what is by far the most important principle of guidance which the student can follow, we will turn to the questions of method that the student need follow in his ordinary work.

With almost all students a difficulty is encountered which hinders them in acquiring any large views as to the world about them. This is due to the fact that they can not make and retain in memory clear pictures of the things they see. They remember words rather than things—in fact, the training in language, which is so large a part of an education, tends ever to diminish the element of visual memory. The first task of the student who would become a naturalist is to take his knowledge from the thing, and to remember it by the mental picture of the thing. In all education in Nature, whether the student is guided by his own understanding or that of the teacher, a first and very continuous aim should be to enforce the habit of recalling very distinct images of all objects which it is desired to remember. To this end the student should practise himself by looking intently upon a landscape or any other object; then, turning away, he should try to recall what he has beheld. After a moment the impression by the sight should be repeated, and the study of the memory renewed. The writer knows by his own experience that even in middle-aged people, where it is hard to breed new habits, such deliberate training can greatly increase the capacity of the memory for taking in and reproducing images which are deemed of importance. Practice of this kind should form a part of every naturalist's daily routine. After a certain time, it need not be consciously done. The movements of thought and action will, indeed, become as automatic as those which the trained fencer makes with his foil.

Along with the habit of visualizing memories, and of storing them without the use of words, the student should undertake to enlarge his powers of conceiving spaces and directions as they exist in the field about him. Among savages and animals below the grade of man, this understanding of spacial relations is very clear and strong. It enables the primitive man to find his way through the trackless forest, and the carrier pigeon to recover his mate and dwelling place from the distance of hundreds of miles away. In civilized men, however, the habit of the home and street and the disuse of the ancient freedom has dulled, and in some instances almost destroyed, all sense of this shape of the external world. The best training to recover this precious capacity will now be set forth.

The student should begin by drawing a map on a true scale, however roughly the work may be done, of those features of the earth about him with which he is necessarily most familiar. The task may well be begun with his own dwelling or his schoolroom. Thence it may be extended so as to include the plan of the neighbouring streets or fields. At first, only directions and distances should be platted. After a time to these indications should be added on the map lines indicating in a general way contours or the lines formed by horizontal planes intersecting the area subject to delineation. After attaining certain rude skill in such work, the student may advantageously make excursions to districts which he can see only in a hurried way. As he goes, he should endeavour to note on a sketch map the positions of the hills and streams and the directions of the roads. A year of holiday practice in such work will, if the tasks occupy somewhere about a hundred hours of his time, serve greatly to extend or reawaken what may be called the topographic sense, and enable him to place in terms of space the observations of Nature which he may make.

In his more detailed work the student should select some particular field for his inquiry. If he be specially interested in geologic phenomena, he will best begin by noting two classes of facts—those exhibited in the rocks as they actually appear in the state of repose as shown in the outcrops of his neighbourhood, and those shown in the active manifestations of geological work, the decay of the rocks and the transportation of their waste, or, if the conditions favour, the complicated phenomena of the seashores.

As soon as the student begins to observe, he should begin to make a record of his studies. To the novice in any science written, and particularly sketched, notes are of the utmost importance. These, whether in words or in drawings, should be made in face of the facts; they should, indeed, be set down at the close of an observation, though not until the observer feels that the object he is studying has yielded to him all which it can at that time give. It is well to remark that where a record is made at the outset of a study the student is apt to feel that he is in some way pledged to shape all he may see to fit that which he has first written. In his early experience as a teacher, the writer was accustomed to have students compare their work of observation and delineation with that done by trained men on the same ground. It now seems to him best for the beginner at first to avoid all such reference of his own work to that of others. So great is the need of developing independent motive that it is better at the outset to make many blunders than to secure accuracy by trust in a leader. The skilful teacher can give fitting words of caution which may help a student to find the true way, but any reference of his undertakings to masterpieces is sure to breed a servile habit. Therefore such comparisons are fitting only after the habit of free work has been well formed. The student who can afford the help of a master, or, better, the assistance of many, such as some of our universities offer, should by all means avail himself of this resource. More than any other science, geology, because of the complexity of the considerations with which it has to deal, depends upon methods of labour which are to a great extent traditional, and which can not, indeed, be well transmitted except in the personal way. In the distinctly limited sciences, such as mathematics, physics, or even those which deal with organic bodies, the methods of work can be so far set forth in printed directions that the student may to a great extent acquire sound ways of work without the help of a teacher.

Although there is a vast and important literature concerning geology, the greater part of it is of a very special nature, and will convey to the beginner no substantial information whatever. It is not until he has become familiar with the field with which he is enabled to deal in the actual way that he can transfer experience thus acquired to other grounds. Therefore beyond the pleasing views which he may obtain by reading certain general works on the science, the student should at the outset of his inquiry limit his work as far as possible to his field of practice, using a good text-book, such as Dana's Manual of Geology, as a source of suggestions as to the problems which his field may afford.

The main aim of the student in this, as in other branches of inquiry, is to gain practice in following out the natural series of actions. To the primitive man the phenomenal world presents itself as a mere phantasmagoria, a vast show in which the things seen are only related to each other by the fact that they come at once into view. The end of science is to divine the order of this host, and the ways in which it is marshalled in its onward movement and the ends to which its march appears to be directed. So far as the student observes well, and thus gains a clear notion of separated facts, he is in a fair way to gather the data of knowledge which may be useful; but the real value of these discernments is not gained until the observations go together, so as to make something with a perspective. Until the store of separate facts is thus arranged, it is merely crude material for thought; it is not in the true meaning science, any more than a store of stone and mortar is architecture. When the student has developed an appetite for the appreciation of order and sources of energy in phenomena, he has passed his novitiate, and becomes one of that happy body of men who not only see what is perceived by the mass of their fellows, but are enabled to look through those chains of action which, when comprehended, serve to rationalize and ennoble all that the senses of man, aided by the instruments which he has devised, tell us concerning the visible world.



INDEX.

AEtna, Mount, 381.

Agriculture, American, 346; in England, winning swamp lands for, 335; recent developments of, 345.

Alaska, changes on the coast of, 96.

Ants taking food underground, 319; work of the, on the soil, 318.

Apsides, revolution of the, 61, 62.

Arabians, chemical experiments of the, 13.

Arches, natural, in cavern districts, 258.

Artesian wells, 258, 259.

Arts, advance of Italian fine, 19.

Asteroids, 53; motions of, about their centres and about the sun, 53.

Astronomers, the solar system and the early, 79.

Astronomy, 31-80; growth of, since the time of Galileo, 33, 34; the first science, 10.

Atmosphere, 97-206; along the tropical belt, 102; as a medium of communication between different regions, 99; deprived of water, containing little heat, 105; beginning of the science of the, 117; counter-trade movements of the, 105; envelope of the earth, 98; expansion of, in a hollow wall during the passage of a storm, 114; heat-carrying power of the, 105; heights to which it extends, 99; in water, 99; movements no direct influence on the surface of the earth, 122; movements of the, qualified by the condition which it encounters, 118; of mountains, 98; of the seashore, 98; of the earth, 98; of the sun, 73; snow as an evidence of, 65; supplying needs of underground creatures, 331; uprushes of, 101, 102; upward strain of the, next the earth, 107; weight and motion of the, 120, 121.

Atmospheric circulation of the soil, 330, 331; envelopes, 97.

Aurora borealis, 168.

Avalanches, 210-213; dreaded, in the Alpine regions, 212; great, in the Swiss Oberland, 211, 212; rocky, 175-177.

Axis, imaginary changes in the earth's, 59; of the earth's rotation, 58; polar, inclined position of, 58; polar, nodding movement of the axes, 54; rotations of the planetary spheres on their axes, 56.

Barometer, causes of changes in the, 117, 118.

Basalts, 309.

Beaches, 93, 142, 144; boulder, 142, 143; pebbly, 142; sand, 144.

Beetles, work of, on the soil, 318, 319.

Belief of the early astronomers about the solar system, 79.

Bergschrund, the, 214.

Birds and mammals contributing to the fertility of the soil, 319.

"Blanketing," 269.

Bogs, climbing, 331-334; lake, 331-333; peat, 334, 335; quaking, 334.

Botany, rapid advance in, 14, 15.

Boulders, 217, 220.

Breakers, 135, 137, 139.

Bridges, natural, 257, 258.

Canals of Mars, 67.

Canon, newly formed river cutting a, 195.

Cataracts, 193.

Caves, 253-258, 261; architecture of, 255-258; hot-water, 261; mammoth cave, 258; stalactites and stalagmites on the roof and floor of, 257.

Chasms, 140, 141.

Chemistry, 6, 12, 14; advance of, 12; modern, evolving from the studies of alchemists, 13, 14.

Chromosphere, 73.

Civilization of the Icelanders, 384.

Cliffs, sea-beaten, 132, 141, 142.

Climate, changes of, due to modifications of the ocean streams, 153; effect of the ocean on the, 147; of the Gulf Stream, 149, 150.

Clouds, 159; formation of, 162, 163; shape of, 163; water of, usually frozen, 207; cloud-making, laws of, 161, 162.

Coast, changes on the Scandinavian, 96; line, effect of tide on the, 145; of Greenland, 226; of New Jersey sinking, 95; marine, changes in, 95.

Cold in Siberia, 243.

Comets, 47, 50; collisions of, 50; kinship of meteorites and, 48; omens of calamity to the ancients, 50; the great, of 1811, 49, 50.

Cones. See under VOLCANOES.

Conflict between religion and science, 20, 22; between the Protestant countries and the followers of science, 20.

Continental shelves, 125.

Continents and oceans, 83; changes in position of, 91; cyclones of the, 111; forms of, 90; proofs that they have endured for many years, 92; shape of, 84, 96.

Coral reefs, 153, 353.

Corona, realm of the, 73.

Craters. See under VOLCANOES.

Crevasse, a barrier to the explorer, 218.

Crevice water, 250.

Curds, 214.

Currents, coral reefs in Florida affecting the velocity of, 153; equatorial, 150; of the Gulf Stream, 147-149; hot and cold, of the sea, 102; ocean, 145; oceanic action of trade winds on, 145; effect on migration of, 157; icebergs indicating, 243; surface, history of, 172; uprushing, near the equator, 106.

Cyclones, 111; cause of, 111; of North America, 111; secondary storms of, 112.

Deltas, 173, 187.

Deposits, vein, 260, 261.

Deserts, interior, 158.

Dew, 159, 160; a concomitant of cloudless skies, 160, and vegetation, 160; formation of, 159-161.

Diablerets, 174.

Diagram of a vein, 260; showing development of swamp, 335; how a portion of the earth's surface may be sunk by faulting, 374; growth of mangroves, 340; the effect of the position of the fulcrum point in the movement of the land masses, 94.

Diameter of our sphere at the equator, 62; of the earth, 82.

Dikes, 192, 293; 305-310; abounding in volcanic cones, 305; cutting through coal, 306; driven upward, 307; formation of, 305, 310; material of, 307, 308; representing movements of softened rock, 309; their relation to volcanic cones, 307; variations of the materials of, 307, 308; waterfalls produced by, 192; zone of, 306.

Dismal Swamp, 95, 333.

Distances, general idea of, 27; good way to study, 27, 28; training soldiers to measure, 28.

Doldrums, 104, 109; doldrum of the equator, 109; of the hurricane, 109.

Drainage, imperfect, of a country affected by glaciers, 242.

Dunes, 123, 124, 325, 326, 387; moulded, 387.

Duration of geological time, 389.

Dust accumulations from wind, in China, 122.

Earth, a flattened sphere, 82; air envelope of the, 98; amount of heat falling from the sun on the, 41; antiquity of the, 391; atmosphere of the, 98; attracting power of the, 127; axis of the rotation of the, 58; composition of the atmosphere of the, 98; crust of the, affected by weight, 93; deviation of the path of the, varied, 61; diameter of the, 82; of the, affected by loss of heat, 131; difference in altitude of the surface of the, 83; discovery that it was globular, 31, 32; effect of imaginary changes in the relations of sun and, 59; effect of the interior heat of the, 309, 310; effect of the sun on the, 60, 61; formerly in a fluid state, 82; imaginary view of the, from the moon, 81; important feature of the surface of the, 83; jarring caused by faults, 367; surface of the, determined by heat and light from the sun, 57; most important feature of the surface of the, 83; motion of the, affecting the direction of trade winds, 103; movements, 366; natural architecture of the, 377; no part of the, exempt from movement, 384; parting of the moon and, 396; path of the, around the sun, 55, 56, 59, 60; revolving from east to west, 103; shrinking of the, from daily escape of heat, 89; soil-covering of the, 343; study of the, 81-96; swaying, 385; tensions, problem of, 371; tremors, caused by chemical changes in the rocks, 385; tropical belt of the, 74; viewed from the surface of the moon, 311, 312; water store of the, 125.

Earthquakes, 277, 278, 280, 356, 358, 370-384, 388-390; accidents of, 358; action of, 356; agents of degradation, 383, 384; basis of, 367; certain limitations to, 380, 381; Charleston, of 1883, 374, 375; countries, architecture in, 381; echoes, 369, 370; damages of, 377, 390; effect of, on the soil, 375; the surface of the earth, 371; formed by riving of fissures, 382; great, occurring where rocks have been disturbed by mountain-building, 381, 382; Herculaneum and Pompeii destroyed by an, 277, 280; Italian, in 1783, 371, 372; important, not connected with volcanic explosions, 381; Jamaica, in 1692, 372, 376; Lisbon, in 1755, 368, 369, 373, 374, 381; maximum swing of, 369; measuring the liability to, 386, 387; mechanism of, 370, 371; method of the study of, followed by Mr. Charles Mallet, 382, 383; Mississippi, in 1811, 373, 374, 380, 381; movement of the earth during, 377; originating from a fault plane, 367, 369, 370; originating from the seas, 358, 375; oscillation of, 376; poised rocks indicating a long exemption from strong, 388; Riobamba, in 1797, 375; shocks of, and their effect upon people, 383; the direct calamities of Nature, 386; waves of, 389.

Earthworms, 317-319; taking food underground, 319.

Eclipses, record of ancient, 130.

Electrical action in the formation of rain and snow, 164.

Elevations of seas and lands, 83.

Energy indestructible, 23.

Envelope, lower, of the sun, 74.

Equator, diameter of our sphere at the, 62; doldrum of the, 109; updraught under the, 102; uprushing current near the, 106.

Equinoxes, precession of the, 61, 62.

Eskers, 221.

Expansion of air contained in a hollow wall during the passage of the storm, 114.

Experiment, illustrating consolidation of disseminated materials of the sun and planets, 40.

Falls. See WATERFALLS.

Fault planes, 382.

Feldspar, 324.

Floods, 180, 197; rarity of, in New England, 121; river, frequent east of Rocky Mountains, 198.

Foehns, 121.

Forests, salicified, 124.

Fossilization, 354-356.

Fulcrum point, 95.

Galactic plane, 45.

Galongoon, eruption of, 294.

Geological work of water, 168-206.

Glacial action in the valleys of Switzerland, 224; periods, 63, 243, 246; in the northern hemisphere, 246; waste, 324.

Glaciation, effect of, in North America, 241; in Central America, 234; South America, 234.

Glaciers, 207-249; action of ice in forming, 230-232; Alaskan, 216; continental, 225, 239, 240; discharge of, 220; exploring, 220; extensive, in Greenland and Scandinavia, 244; former, of North America, 232, 234; map of, and moraines near Mont Blanc, 217; motions of, 213; retreat of the, 228, 230, 235; secrets of the under ice of, 221; speed of a, 224; study of, in the Swiss valleys, 222; testimony of the rocks regarding, 228; when covered with winter snows, 216; valley, 216.

Gombridge, 1830, 74.

Gravitation, law of, 4.

Greeks' idea of the heavens, 31; not mechanically inventive, 22.

Gulf Stream, current of the, 147.

Heat, amount of, daily escaping from the earth, 89; amount of, falling from the sun on the earth, 41; belief of the ancients regarding, 42; dominating effect on air currents of tropical, 104; energy with which it leaves the sun, 41; internal, of the earth, 88, 89; of the earth's interior, 309, 310; sun, effect on the atmosphere of the, 100; Prof. Newcomb's belief regarding the, of the sun, 52; radiation of the earth's, causing winds, 101; solar, 41; tropical, and air currents, 104.

Hills, sand, 123.

Horizontal pendulum, 384.

Horse latitudes, 104.

"Horses," 261.

Hurricanes, 107, 110, 317; commencement of, 107; doldrum of, 109; felt near the sea, 110; in the tropics, 110.

Hypothesis, nebular, 34, 35, 39, 52, 56; working, 4, 5.

Ice action, effect of intense, 222, 223; in forming glaciers, 230, 232; recent studies in Greenland of, 239; depth of, in Greenland, 227; effect of, on river channels, 196; effect of, on stream beds, 196; expanding when freezing, 237; epoch, 92, 93, 246; floating, 242; made soils rarely fertile, 241; mass, greatest, in Greenland, 226, 227; moulded by pressure, 215; streams, continental, 225, 226; of the mountains, 225; of the Himalayan Mountains, 234.

Icebergs, 242, 243; indicating oceanic currents, 243.

Iceland, volcanic eruptions in, 297, 298.

Instruments, first, astronomical, 10, 11.

Inventions, mechanical, aiding science, 22.

Islands, 84, 272; continental, 84; in the deeper seas made up of volcanic ejections, 272; volcanic, 272.

Jack-o'-lantern, 167.

Jupiter, gaseous wraps of, 97; path of the earth affected by, 59, 60; the largest planet of the sun, 69.

Kames, 325.

Kant, Immanuel, and nebular hypothesis, 34.

Kaolin, 324.

Klondike district, cold in, 243, 244.

Krakatoa, eruption of, 298-300; effect of, on the sea, 299; effect of, on the sun, 300.

Lacolites, 306.

Lacustrine beds, 351.

Lagoons, salt deposits found in, 200.

Lake basins, formation of, 200, 201; bogs, 331, 333, 334; deposits, 350, 351.

Lakes, 199-206; effect of, on the river system, 205; fresh-water, 145; formed from caverns, 202; great, changing their outlets, 205; of extinct volcanoes, 203; temporary features of the land, 203; volcanic, 203.

Lands, great, relatively unchangeable, 96; table, 91; movements resulting in change of coast line, 351, 352; shape of the seas and, 83, 84; accounting for the changes in the attitude of the, 95; and water, divisions of, 84; dry, surface of, 85; general statement as to the division of the, 83, 84; surface, shape of the, 85; triangular forms of great, 90.

Latitudes, horse, troublesome to mariners, 104.

Laplace and nebular hypothesis, 34.

Lava, 266-268, 270, 271, 292, 293, 295, 296, 303, 304; flow of, invading a forest, 268; from Vesuvius, 293; of 1669, 295, 296; temperature of, 295, 296; incipient, 304; outbreaks of, 292, 303; stream eaves, 292, 293.

Law, natural, Aristotle and, 3; of gravitation, 4; of the conservation of energy, 23.

Leaves, radiation of, 160.

Length of days affected by tidal action, 131.

Level surfaces, 91.

Life, organic, evolution of, 15, 16.

Light, belief of the ancients regarding, 42.

Lightning, 24, 164-168; noise from, 166; proceeding from the earth to the clouds, 165; protection of buildings from, 165; stroke, wearing-out effect of, 165.

Limestones, 353, 357, 358, 360, 364; formation of, 357, 360.

Lisbon, earthquake of, 1755, 368, 369.

Lowell, Mr. Percival, observations on Venus, 64.

Lunar mountains near the Gulf of Iris, 397.

Mackerel sky, 35.

Mallet, Mr. Charles, and the study of earthquakes, 382, 383.

Man as an inventor of tools, 10.

Mangroves, 340; diagram showing mode of growth, 340; marshes of, 339.

Map of glaciers and moraines near Mont Blanc, 217; of Ipswich marshes, 338.

Mapping with contour lines, 27.

Maps, desirable, for the study of celestial geography, 77; geographic sketch, 26, 27.

Marching sands jeopardizing agriculture, 123.

Marine animals, sustenance of, 361-363; deposits, 325-327, 349, 356; marshes, 336-340; waves caused by earthquakes, 387.

Mars, 65-67, 84, 97; belief that it has an atmosphere, 65; canals of, 67; gaseous wraps of, 97; more efficient telescopes required for the study of, 67; nearer to the earth than other planets, 65.

Marshes, mangrove, 339; map of Ipswich, 338; marine, 336-340; deposits found in, 336; of North America, 337; on the coast of New England, 339; phenomena of, 167, 168; tidal, good earth for tillage, 337; tidal, of North America, 340.

Mercury, 55, 63, 78; nearest to the sun, 63; time in which it completes the circle of its year, 55.

Meteorites, 47, 48; kinship of comets and, 48.

Meteors, 47; falling, 47; composition of, 48; flashing, 39, 40, 47; speed of, 47; inflamed by friction with air, 99.

Methods in studying geology, 400.

Milky Way, 45; voyage along the path of the, 44, 45.

Mineral crusts, 328, 329; deposits, 308.

Moon, 38, 395-400; absence of air and water on the, 399; attended by satellites, 57; attraction which it exercises on the earth, 62; curious feature of the, 397; destitute of gaseous or aqueous envelope, 397; diameter of the, 399; imaginary view of the earth from the, 81; "libration" of the, 398; made up of circular depressions, 396, 397; movements of the, 78; no atmosphere in the, 97; parting of the earth and, 396; position of the, in relation to the earth, 62; tidal action and the, 131; tides of the, 126, 127; why does the sun not act in the same manner as the, 78.

Moraines, 216, 218, 229, 230; map of glaciers and, near Mont Blanc, 217; movements of the, 216-218; terminal, 228.

Moulin, 219.

Mount AEtna, 288-310; lava yielding, 290, 293, 294; lava stream caves of, 292, 293; more powerful than Vesuvius, 297; peculiarities of, 291, 292; size of, 289-291; turning of the torrents of, 295.

Mountain-building, 90-93, 304; folding, 86, 87, 90, 365; attributed to cooling of the earth, 88; growth, 392; Swiss falls, 174; torrents, energy of, 177.

Mountains, 85, 86, 89, 90-93; 174-178; form and structure of, 86; partly caused by escape of heat from the earth, 89; sections of, 87.

Mount Nuova, formation of, 284.

Mount Vesuvius, 263-285, 288, 289, 293, 302, 381; description of the eruption of, in A.D. 79, 277-280; diagrammatic sections through, showing changes in the form of the cone, 283; eruption of, in 1056, 281; in 1882-'83, 264, 266; eruption of, in 1872, 282; eruptions of, increased since 1636, 282; flow of lava from, 285; likely to enter on a period of inaction, 282, 283; outbreak of, in 1882-'83, 264, 266.

Naples, prosperity of the city, 289.

Nebular hypothesis, 34, 35, 39, 52.

Neptune, 70.

Neve, the, 214; no ice-cutting in the region of the, 224.

Newcomb's (Prof.) belief regarding the heat of the sun, 52.

Niagara Falls, 191, 192, 204; cutting back of, 204.

North America, changes in the form of, 91, 92; triangular form of, 90.

Ocean, average depth of the, 89; climatal effect of the, 147; currents, 145; effect of, on migration, 156; effect of, on organic life, 154; floor, 85, 93; hot and cold currents of the, 102; sinking of the, 93, 94; the laboratory of sedimentary deposits, 351; depth of the, 89, 126.

Oceanic circulation, effect of, on the temperature, 152.

Oceans and continents, 83.

Orbit, alterations of the, and the seasons, 60, 61; changing of the, 59-63; shape of the, 61-63.

Organic life, 315, 317, 321, 352, 353, 363; action of, on the soil, 317, 321; advantages of the shore belt to, 363; development of in the sea, 352, 353; effect of ocean currents on, 154; processes of, in the soil, 315; decay of, in the earth, 321.

Orion, 46.

Oscillations of the shores of the Bay of Naples, 287.

Oxbow of a river, 182, 183.

Oxbows and cut-off, 182.

Pebbles, action of seaweeds on, 143; action of the waves on, 142, 144.

Photosphere, 74.

Plains, 86; alluvial, 91, 179, 182, 184-186, 325; history of, 91; sand, 325.

Planets, 38; attended by satellites, 57; comparative sizes of the, 68; experiments illustrating consolidation of disseminated materials of the sun and, 40; gaseous wraps of, 97; important observations by the ancients of fixed stars and planets, 43; movements of, 57-61; outer, 78; table of relative masses of sun and, 77.

Plant life in the Sargassum basins, 156.

Plants and animals, protection of, by mechanical contrivances, 364; and trees, work of the roots of, on the soil, 316, 317; water-loving, 181; forming climbing bogs, 332.

Polar axes, nodding movement of, 54.

Polar snow cap, 66.

Polyps, 155, 353.

Pools, circular, 203.

Prairies, 340, 342.

Radiation of heat, 159.

Rain, 152, 156, 164, 168, 170, 328, 330; circuit of the, 156-168; drops, force of, 169, 170; spheroidal form of, 170; electrical action in the formation of snow and, 164; work of the, 171.

Realm, unseen solar, 75.

Reeds, 332.

Religion, conflict between science and, 20, 22; struggle between paganism and, 21.

Rivers and debris, 183; changes in the course of, in alluvial plain, 182; deposition of, accelerated by tree-planting, 181; great, always clear, 205; inundation of the Mississippi, eating away land, 182; muds, 222; newly formed, cutting a canon, 195; of snow-ice, 211; origin of a normal, 173; oxbow of a, 182,183; sinking of, 199; swinging movement of, 179-181; river-valleys, 193, 194; diversity in the form of 188-191.

Rocks, 145; accidents from falling, 174; cut away by sandstones, 188; divided by crevices, 252; duration of events recorded in, 389, 390, ejection of, material, 311; falling of, 174-176; formation of, 262, 263; from the present day to the strata of the Laurentian, 390; migration of, 291; poised, indicating a long exemption from strong earthquakes, 388; rents in, 252, 253; stratification of, 349, 350, 352, 365, 390; testimony of the, in regard to glaciers, 228; under volcanoes, 303; variable elasticity of, 366; vibration of, 367, 368; rock-waste, march of the, 343; water, 250, 267.

Rotation of the earth affected by tides, 130; of the planetary spheres on their axes, 56.

Salicified forests, 124.

Salt deposits formed in lagoons, 200; found in lakes, 199-200.

Sand bars, 183; endurance of, against the waves, 145; hills, travelling of, 123; marching, 123; silicious stones cutting away rooks, 188.

Satellites, 53, 54; motions of, about their centres and about the sun, 53, 54.

Saturn, 38, 53, 57, 396; cloud bands of, 70; gaseous wraps of, 97; path of the earth affected by, 59, 60.

Savages, primitive, students of Nature, 1.

Scandinavia, changes on the coasts of, 96.

Science, advance of, due to mechanical inventions, 22; astronomy beginning with, 10; chemical, characteristics of, 14; conflict between religion and, 20, 22; conflict between the Roman faith and, 20; mechanical inventions as aids to, 22, 23; modern and ancient, 4; natural, 5, 6; of botany in Aristotle's time, 14; of physiology, 15; of zooelogy in Aristotle's time, 14; resting practically on sight, 10.

Scientific development, historic outlines of, 17; tools used in measuring and weighing, as an aid to vision, 12.

Sea, battering action of the, 140; coast ever changing, 385, 386; effect of volcanic eruptions on the, 299; floor deposits of the, affected by volcanoes, 360, 361; in receipt of organic and mineral matter, 359; hot and cold currents of the, 102; littoral zone of the, 351, 352; puss, 142; rich in organic life, 352, 353; solvent action of the, 361; strata, formation of, 354; water, minerals in, 185; weeds, 155, 156.

Seas, dead, originally living lakes, 200; water of, buoyant, 199; eventually the seat of salt deposits, 199-201; general statement as to division of, 83, 84; shape of the, 83, 84.

Seashore, air of the, 98.

Seasons, changing the character of the, 61, 62.

Sense of hearing, 9,10; of sight, 10; of smell, 9, 10; of taste, 9, 10; of touch, 9, 10.

Seracs, 214.

Shocks, earthquake. See under EARTHQUAKES.

Shore lines, variation of, 83, 84.

Shores, cliff, 138-142.

Sink holes, 202; in limestone districts, 253, 254.

Skaptar, eruption of, 297, 298; lava from the eruption of, 298.

Sky, mackerel, 35.

Snow, 207-225, 244; as an evidence of atmosphere, 65; blankets, early flowers beginning to blossom under, 208; covering, difference between an annual and perennial, 210; effect of, on plants, 208; electrical action in the formation of rain and, 164; flakes, formation of, 164; red, 210; slides, 210; slides, phenomena of, 210, 211.

Soil, alluvial, 321, 322; atmospheric circulation of, 330, 331; conditions leading to formation of, 313, 331; continuous motion of the, 314; covering of the earth, 343; decay of the, 314, 315; degradation of the, 344-348; means for correcting, 346-348; destruction in grain fields greater than the accumulation, 344; developing on lava and ashes an interesting study, 343; development of, in desert regions, 340; effect of animals and plants on the, 317-320; effect of earthquakes on the, 375; fertility of the, distinguished from the coating, 344, 345; fertility of, affected by rain, 327; formation of, 314-321; glacial, characteristics of, 324; glaciated, 323, 324; irrigation of the, 328-330; local variation of, 327; mineral, 321; of arid regions fertile when subjected to irrigation, 341; of dust or blown sand, 321; of immediate derivation, 321, 322; phenomena, 313; processes of organic life in the, 315; variation in, 321-331; vegetation protecting the, 316, 317; washing away of the, 346, 347; winning, from the sea, 337; work of ants on the, 318; tiller, duty of the, 348.

Solar bodies, general conditions of the, 63-71; forces, action of, on the earth, 349; system, 52, 56; independent from the fixed stars system, 43; original vapour of, 52, 53; singular features of our, 68; tide, 127.

Spheres, difference in magnitude of, 51; motions of the, 50, 51; planetary, rotation of, on their axes, 56.

Spots, sun, 72.

Spouting horn, 141.

Springs, formation of small, 252.

Stalactitization, 256.

Stalagmites and stalactites on the roof and floor of a cavern, 257.

Stars as dark bodies in the heavens, 47; discovery of Fraunhofer and others on, 23, 38; double, 39; and tidal action, 131; earliest study of, 10; fixed, important observations by the ancients of planets and, 43; not isolated suns, 38, 39; variation in the light of, 46; limit of, seen by the naked eye, 11; revolution of one star about another, 46, 47; shooting, 47; speed of certain, 51; study of, 31-80; sudden flashing forth of, due to catastrophe, 46; voyage through the, 44, 45; star, wandering, 74.

Stellar realm, 31-80.

Storms, circular, 111; desert, 121, 122; expansion of air contained in a hollow wall during the passage of, 114; great principle of, 105, 106; in the Sahara, 121; lightning, more frequent in summer, 167; paths of, 115; secondary, of cyclones, 112; spinning, 115; thunder, 165-167; whirling, 106, 124; whirling peculiarity of, 108, 109.

Strabo, writings of, 18.

Sun, atmosphere of the, 73; constitution of the, 72; distance of the earth from the, 29; effect from changes in the, and earth, 59; envelope of the, 73, 74, 97; experiments illustrating consolidation of disseminated materials of planets and, 40; finally, dark and cold, 42; formation of the eight planets of the, 53; heat leaving the, 41; heat of the, 76; imaginary journey from the, into space, 44; mass of the, 76, 77; path of the earth around the, 55; physical condition of the, 71; Prof. Newcomb's belief regarding the heat of the, 52; spots, 75; abundant at certain intervals, 72; difficulty in revealing cause of, 75; structure of the, a problem before the use of the telescope, 72; table of relative masses of, and planets, 77; three stages in the history of the, 71; tides, 126; why does it not act in the same manner as the moon? 78.

Surfaces, level, 90.

Surf belt, swayings of the, 137.

Swamps, diagram showing remains of, 335; Dismal Swamp, 95, 333; drainage of, 334, 335; fresh-water, 334, 335; phenomena of, 167, 168.

Table-lands, 91.

Table of relative masses of sun and planets, 77.

Telescopes, 11, 12, 45; first results of, 72; power of, 11; revelations of, 45.

Temperature, effects of, produced by vibration, 42; in the doldrum belt, 118; of North America, 118; of the Atlantic Ocean, 118.

Tempests, rate of, 99, 100.

Thunder, 166; more pronounced in the mountains, 166.

Thunderstorms, 165, 166; distribution of, 166, 167.

Tidal action, recent studies of, 131, 132; marshes of North America, 340.

Tides, carving channels, 129; effecting the earth's rotation, 130; effect of, on marine life, 130; height of, 128, 129; moon and sun, 126, 127; normal run of the, 127; production of, 131; of the trade winds, 150; solar, 127; travelling of, 127, 128.

Tillage introducing air into the pores of the soil, 331.

Tornadoes, 112, 113, 317; development of, 113; effect of, on buildings, 113; fiercest in North America, 113; length of, 115; resemblance of, to hurricanes, 115; upsucking action of, 114, 115.

Torrents, 177-179, 204.

Trade winds. See under WINDS.

Training in language, diminishing visual memory, 401; soldiers to measure distances, 28; to measure intervals of time, 28; for a naturalist, 25-29.

Tunnels, natural, 257.

Uranus, 70.

Valley of Val del Bove formed from disturbances of Mount AEtna, 294.

Valleys, diversity in the form of river, 188-191; river, 193.

Vapour, 156, 157, 159, 163; gravitative attraction of, 34, 35; nebular theory of, 52, 53; original, of the solar system, 52, 53.

Vegetation, and dew, 160; in a measure, independent of rain, 160; protecting the soil, 316, 317.

Vein, diagram of a, 260.

Venus, 64, 78; recent observations of, by Mr. Percival Lowell, 64.

Vesuvian system, study of the, 285.

Vesuvius. See MOUNT VESUVIUS.

Visualizing memories, 402, 403.

Volcanic action, 268-276.

Volcanic eruption of A.D. 79, 288; important facts concerning, 276-279; islands, 272; lava a primary feature in, 266; observations of, made from a balloon, 301; peaks along the floor of the sea, 272, 273; possibility of throwing matter beyond control of gravitative energy, 300.

Volcanoes, 125, 203, 263; abounding on the sea floor, 302; accidents from eruptions of, 288; along the Pacific coast, 271; ash showers of, maintaining fertility of the soil, 289; distribution of, 271; eruption of, 286-294, 368; explosions from, coming from a supposed liquid interior of the earth, 275; exporting earth material, 310; little water, 375; Italian, considered collectively, 296, 297; Neapolitan eruptions of and the history of civilization, 288; subsidence of the earth after eruption of, 287, 291; origin of, 263-274; phenomena of, 263-267; submarine, 301; travelling of ejections from, 287, 288.

Waters, crevice, 250; of the earth, 250, 251; cutting action of, 117, 192; drift, from the poles, 151; journey of, from the Arctic Circle to the tropics, 151, 152; dynamic value of, 171; expansion of, in rocks, 270; geological work of, 168-206; in air, 99; of the clouds usually frozen, 207; pure, no power for cutting rocks, 204; rock, 250, 263; sea, minerals in, 185; store of the earth, 125; system of, 125, 156; tropical, 151; velocity of the, under the equator, 150; wearing away rocks, 178, 179; underground, carrying mineral matter to the sea, 193; chemical changes of, leading to changes in rock material, 262, 263; effect of carbonic-acid gas on, 251; operations of the, 126; wearing away rocks, 178, 179; work of, 250.

Waterfalls, 189-193; cause of, 191; the Yosemite, 192; Niagara, 191, 192; numerous in the torrent district of rivers, 192; produced by dikes, 192; valuable to manufactures, 192, 193.

Waterspouts, 115, 116; atmospheric cause of, 116; firing at, 116; life of a, 116; picturesqueness of, 116; the water of fresh, 117.

Waves, 128, 129, 132, 145; action of friction on, 135, 136; break of the, 136; endurance of sand against the, 145; force of, 133, 136, 139; marine, caused by earthquakes, 387; of earthquakes, 389; peculiar features in the action of, 137; size of, 137, 138; stroke of the, 144; surf, 135; tidal height of, 132; undulations of, 132; wind, 132; wind influence of, on the sea, 134, 135; wind-made, 128.

Ways and means of studying Nature, 9.

Weeds of the sea, 155.

Well, artesian, 258, 259.

Whirling of fluids and gas, 36, 37.

Whirlwinds in Sahara, 121.

Will-o'-the-wisp, 167.

Winds, 101, 110, 122, 317; effect of sand, 122; hurricane, 110; illustration of how they are produced, 101; in Martha's Vineyard, 120; of the forests, work of the, 317; of tornadoes, effect of, 113; on the island of Jamaica, 119, 120; regimen of the, 119; variable falling away in the nighttime, 100; trade, 102-105; 145, 146, 150; action of, on ocean currents, 145: affected by motion of the earth, 103; belt, motion of the ocean in, 146; flow and counter-flow of the, 150; tide of the, 150; uniform condition of the, 102; waves, work of, 132, 134, 135.

Witchcraft, belief of, in the early ages, 21.

Zooelogy, rapid advance in, 14, 15.

THE END