[1] Dana, Manual of Geology, third edition, p. 794

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On the western side of North America the work of mountain-building was, indeed, on the grandest scale. For long ages and through a succession of geological epochs, sedimentation had proceeded so that the accumulations of Palaeozoic and Mesozoic times had collected in the geosyncline formed by their own ever increasing weight. The site of the future Laramide range was in late Cretaceous times occupied by some 50,000 feet of sedimentary deposits; but the limit had apparently been attained, and at this time the Laramide range, as well as its southerly continuation into the United States, the Rockies, had their beginning. Chamberlin and Salisbury[1] estimate that the height of the mountains developed in the Laramide range at this time was 20,000 feet, and that, owing to the further elevation which has since taken place, from 32,000 to 35,000 feet would be their present height if erosion had not reduced them. Thus on either side of the American continent we have the same forces at work, throwing up mountain ridges where the sediments had formerly been shed into the ocean.

These great events are of a rhythmic character; the crust, as it were, pulsating under the combined influences of sedimentation and denudation. The first involves downward movements under a gathering load, and ultimately a reversal of the movement to one of upheaval; the second factor, which throughout has been in

[1] Chamberlin and Salisbury, Geology, 1906, iii., 163.

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operation as creator of the sediments, then intervenes as an assailant of the newly-raised mountains, transporting their materials again to the ocean, when the rhythmic action is restored to its first phase, and the age-long sequence of events must begin all over again.

It has long been inferred that compressive stress in the crust must be a primary condition of these movements. The wvork required to effect the upheavals must be derived from some preexisting source of energy. The phenomenon—intrinsically one of folding of the crust—suggests the adjustment of the earth-crust to a lessening radius; the fact that great mountain-building movements have simultaneously affected the entire earth is certainly in favour of the view that a generally prevailing cause is at the basis of the phenomenon.

The compressive stresses must be confined to the upper few miles of the crust, for, in fact, the downward increase of temperature and pressure soon confers fluid properties on the medium, and slow tangential compression results in hydrostatic pressure rather than directed stresses. Thus the folding visible in the mountain range, and the lateral compression arising therefrom, are effects confined to the upper parts of the crust.

The energy which uplifts the mountain is probably a surviving part of the original gravitational potential energy of the crust itself. It must be assumed that the crust in following downwards the shrinking subcrustal magma, develops immense compressive stresses in

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its materials, vast geographical areas being involved. When folding at length takes place along the axis of the elongated syncline of deposition, the stresses find relief probably for some hundreds of miles, and the region of folding now becomes compressed in a transverse direction. As an illustration, the Laramide range, according to Dawson, represents the reduction of a surface-belt 50 miles wide to one of 25 miles. The marvellous translatory movements of crustal folds from south to north arising in the genesis of the Swiss Alps, which recent research has brought to light, is another example of these movements of relief, which continue to take place perhaps for many millions of years after they are initiated.

The result of this yielding of the crust is a buckling of the surface which on the whole is directed upwards; but depression also is an attendant, in many cases at least, on mountain upheaval. Thus we find that the ocean floor is depressed into a syncline along the western coast of South America; a trough always parallel to the ranges of the Andes. The downward deflection of the crust is of course an outcome of the same compressive stresses which elevate the mountain.

The fact that the yielding of the crust is always situated where the sediments have accumulated to the greatest depth, has led to attempts from time to time of establishing a physical connexion between the one and the other. The best-known of these theories is that of Babbage and Herschel. This seeks the connexion in the rise of the

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geotherms into the sinking mass of sediment and the consequent increase of temperature of the earth-crust beneath. It will be understood that as these isogeotherms, or levels at which the temperature is the same, lie at a uniform distance from the surface all over the Earth, unless where special variations of conductivity may disturb them, the introduction of material pressed downwards from above must result in these materials partaking of the temperature proper to the depth to which they are depressed. In other words the geotherms rise into the sinking sediments, always, however, preserving their former average distance from the surface. The argument is that as this process undoubtedly involves the heating up of that portion of the crust which the sediments have displaced downwards, the result must be a local enfeeblement of the crust, and hence these areas become those of least resistance to the stresses in the crust.

When this theory is examined closely, we see that it only amounts to saying that the bedded rocks, which have taken the place of the igneous materials beneath, as a part of the rigid crust of the Earth, must be less able to withstand compressive stress than the average crust. For there has been no absolute rise of the geotherms, the thermal conductivities of both classes of materials differing but little. Sedimentary rock has merely taken the place of average crust-rock, and is subjected to the same average temperature and pressure prevailing in the surrounding crust. But are there any grounds for the

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assumption that the compressive resistance of a complex of sedimentary rocks is inferior to one of igneous materials? The metamorphosed siliceous sediments are among the strongest rocks known as regards resistance to compressive stress; and if limestones have indeed plastic qualities, it must be remembered that their average amount is only some 5 per cent. of the whole. Again, so far as rise of temperature in the upper crust may affect the question, a temperature which will soften an average igneous rock will not soften a sedimentary rock, for the reason that the effect of solvent denudation has been to remove those alkaline silicates which confer fusibility.

When, however, we take into account the radioactive content of the sediments the matter assumes a different aspect.

The facts as to the general distribution of radioactive substances at the surface, and in rocks which have come from considerable depths in the crust, lead us to regard as certain the widespread existence of heat-producing radioactive elements in the exterior crust of the Earth. We find, indeed, in this fact an explanation—at least in part—of the outflow of heat continually taking place at the surface as revealed by the rising temperature inwards. And we conclude that there must be a thickness of crust amounting to some miles, containing the radioactive elements.

Some of the most recent measurements of the quantities of radium and thorium in the rocks of igneous origin—e.g. granites, syenites, diorites, basalts, etc., show that the

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radioactive heat continually given out by such rocks amounts to about one millionth part of 0.6 calories per second per cubic metre of average igneous rock. As we have to account for the escape of about 0.0014 calorie[1] per square metre of the Earth's surface per second (assuming the rise of temperature downwards, i.e. the "gradient" of temperature, to be one degree centigrade in 35 metres) the downward extension of such rocks might, prima facie, be as much as 19 kilometres.

About this calculation we have to observe that we assume the average radioactivity of the materials with which we have dealt at the surface to extend uniformly all the way down, i.e. that our experiments reveal the average radioactivity of a radioactive crust. There is much to be said for this assumption. The rocks which enter into the measurements come from all depths of the crust. It is highly probable that the less silicious, i.e. the more basic, rocks, mainly come from considerable depths; the more acid or silica-rich rocks, from higher levels in the crust. The radioactivity determined as the mean of the values for these two classes of rock closely agrees with that found for intermediate rocks, or rocks containing an intermediate amount of silica. Clarke contends that this last class of material probably represents the average composition of the Earth's crust so far as it has been explored by us.

[1] The calorie referred to is the quantity of heat required to heat one gram of water, i.e. one cubic centimetre of water—through one degree centigrade.

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It is therefore highly probable that the value found for the mean radioactivity of acid and basic rocks, or that found for intermediate rocks, truly represents the radioactive state of the crust to a considerable depth. But it is easy to show that we cannot with confidence speak of the thickness of this crust as determinable by equating the heat outflow at the surface with the heat production of this average rock.

This appears in the failure of a radioactive layer, taken at a thickness of about 19-kilometres, to account for the deep-seated high temperatures which we find to be indicated by volcanic phenomena at many places on the surface. It is not hard to show that the 19-kilometre layer would account for a temperature no higher than about 270 deg. >C. at its base.

It is true that this will be augmented beneath the sedimentary deposits as we shall presently see; and that it is just in association with these deposits that deep-seated temperatures are most in evidence at the surface; but still the result that the maximum temperature beneath the crust in general attains a value no higher than 270 deg. C. is hardly tenable. We conclude, then, that some other source of heat exists beneath. This may be radioactive in origin and may be easily accounted for if the radioactive materials are more sparsely distributed at the base of the upper crust. Or, again, the heat may be primeval or original heat, still escaping from a cooling world. For our present purpose it does not much matter which view

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we adopt. But we must recognise that the calculated depth of 19 kilometres of crust, possessing the average radioactivity of the surface, is excessive; for, in fact, we are compelled by the facts to recognise that some other source of heat exists beneath.

If the observed surface gradient of temperature persisted uniformly downwards, at some 35 kilometres beneath the surface there would exist temperatures (of about 1000 deg. C.) adequate to soften basic rocks. It is probable, however, that the gradient diminishes downwards, and that the level at which such temperatures exist lies rather deeper than this. It is, doubtless, somewhat variable according to local conditions; nor can we at all approximate closely to an estimate of the depth at which the fusion temperatures will be reached, for, in fact, the existence of the radioactive layer very much complicates our estimates. In what follows we assume the depth of softening to lie at about 40 kilometres beneath the surface of the normal crust; that is 25 miles down. It is to be observed that Prestwich and other eminent geologists, from a study of the facts of crust-folding, etc., have arrived at similar estimates.[1] As a further assumption we are probably not far wrong if we assign to the radioactive part of this crust a thickness of about 10 or 12 kilometres; i.e. six or seven miles. This is necessarily a rough approximation only; but the conclusions at which

[1] Prestwich, Proc. Royal Soc., xii., p. 158 et seq.

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we shall arrive are reached in their essential features allowing a wide latitude in our choice of data. We shall speak of this part of the crust as the normal radioactive layer.

An important fact is evolved from the mathematical investigation of the temperature conditions arising from the presence of such a radioactive layer. It is found that the greatest temperature, due to the radioactive heat everywhere evolved in the layer—i.e. the temperature at its base—is proportional to the square of the thickness of the layer. This fact has a direct bearing on the influence of radioactivity upon mountain elevation; as we shall now find.

The normal radioactive layer of the Earth is composed of rocks extending—as we assume—approximately to a depth of 12 kilometres (7.5 miles). The temperature at the base of this layer due to the heat being continually evolved in it, is, say, t1 deg.. Now, let us suppose, in the trough of the geosyncline, and upon the top of the normal layer, a deposit of, say, 10 kilometres (6.2 miles) of sediments is formed during a long period of continental denudation. What is the effect of this on the temperature at the base of the normal layer depressed beneath this load? The total thickness of radioactive rocks is now 22 kilometres. Accordingly we find the new temperature t2 deg., by the proportion t1 deg. : t2 deg. :: 12 deg. : 22 deg. That is, as 144 to 484. In fact, the temperature is more than trebled. It is true we here assume the radioactivity of the sediments

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and of the normal crust to be the same. The sediments are, however, less radioactive in the proportion of 4 to 3. Nevertheless the effects of the increased thickness will be considerable.

Now this remarkable increase in the temperature arises entirely from the condition attending the radioactive heating; and involves something additional to the temperature conditions determined by the mere depression and thickening of the crust as in the Babbage-Herschel theory. The latter theory only involves a shifting of the temperature levels (or geotherms) into the deposited materials. The radioactive theory involves an actual rise in the temperature at any distance from the surface; so that the level in the crust at which the rocks are softened is nearer to the surface in the geosynclines than it is elsewhere in the normal crust (Pl. XV, p. 118).

In this manner the rigid part of the crust is reduced in thickness where the great sedimentary deposits have collected. A ten-kilometre layer of sediment might result in reducing the effective thickness of the crust by 30 per cent.; a fourteen-kilometre layer might reduce it by nearly 50 per cent. Even a four-kilometre deposit might reduce the effective resistance of the crust to compressive forces, by 10 per cent.

Such results are, of course, approximate only. They show that as the sediments grow in depth there is a rising of the geotherm of plasticity—whatever its true temperature may be—gradually reducing the thickness of that part

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of the upper crust which is bearing the simultaneously increasing compressive stresses. Below this geotherm long-continued stress resolves itself into hydrostatic pressure; above it (there is, of course, no sharp line of demarcation) the crust accumulates elastic energy. The final yielding and flexure occur when the resistant cross-section has been sufficiently diminished. It is probable that there is also some outward hydrostaitic thrust over the area of rising temperature, which assists in determining the upward throw of the folds.

When yielding has begun in any geosyncline, and the materials are faulted and overthrust, there results a considerably increased thickness. As an instance, consider the piling up of sediments over the existing materials of the Alps, which resulted from the compressive force acting from south to north in the progress of Alpine upheaval. Schmidt of Basel has estimated that from 15 to 20 kilometres of rock covered the materials of the Simplon as now exposed, at the time when the orogenic forces were actively at work folding and shearing the beds, and injecting into their folds the plastic gneisses coming from beneath.[1] The lateral compression of the area of deposition of the Laramide, already referred to, resulted in a great thickening of the deposits. Many other cases might be cited; the effect is always in some degree necessarily produced.

[1] Schmidt, Ec. Geol. Helvelix, vol. ix., No. 4, p. 590

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If time be given for the heat to accumulate in the lower depths of the crushed-up sediments, here is an additional source of increased temperature. The piled-up masses of the Simplon might have occasioned a rise due to radioactive heating of one or two hundred degrees, or even more; and if this be added to the interior heat, a total of from 800 deg. to 1000 deg. might have prevailed in the rocks now exposed at the surface of the mountain. Even a lesser temperature, accompanied by the intense pressure conditions, might well occasion the appearances of thermal metamorphism described by Weinschenk, and for which, otherwise, there is difficulty in accounting.[1]

This increase upon the primarily developed temperature conditions takes place concurrently with the progress of compression; and while it cannot be taken into account in estimating the conditions of initial yielding of the crust, it adds an element of instability, inasmuch as any progressive thickening by lateral compression results in an accelerated rise of the goetherms. It is probable that time sufficient for these effects to develop, if not to their final, yet to a considerable extent, is often available. The viscous movements of siliceous materials, and the out-pouring of igneous rocks which often attend mountain elevation, would find an explanation in such temperatures.

[1] Weinschenk, Congres Geol. Internat., 1900, i., p. 332.

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There is no more striking feature of the part here played by radioactivity than the fact that the rhythmic occurrence of depression and upheaval succeeding each other after great intervals of time, and often shifting their position but little from the first scene of sedimentation, becomes accounted for. The source of thermal energy, as we have already remarked, is in fact transported with the sediments—that energy which determines the place of yielding and upheaval, and ordains that the mountain ranges shall stand around the continental borders. Sedimentation from this point of view is a convection of energy.

When the consolidated sediments are by these and by succeeding movements forced upwards into mountains, they are exposed to denudative effects greatly exceeding those which affect the plains. Witness the removal during late Tertiary times of the vast thickness of rock enveloping the Alps. Such great masses are hurried away by ice, rivers, and rain. The ocean receives them; and with infinite patience the world awaits the slow accumulation of the radioactive energy beginning afresh upon its work. The time for such events appears to us immense, for millions of years are required for the sediments to grow in thickness, and the geotherms to move upwards; but vast as it is, it is but a moment in the life of the parent radioactive substances, whose atoms, hardly diminished in numbers, pursue their changes while the mountains come and go, and the

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rudiments of life develop into its highest consummations.

To those unacquainted with the results of geological investigation the history of the mountains as deciphered in the rocks seems almost incredible.

The recently published sections of the Himalaya, due to H. H. Hayden and the many distinguished men who have contributed to the Geological Survey of India, show these great ranges to be essentially formed of folded sediments penetrated by vast masses of granite and other eruptives. Their geological history may be summarised as follows

The Himalayan area in pre-Cambrian times was, in its southwestern extension, part of the floor of a sea which covered much of what is now the Indian Peninsula. In the northern shallows of this sea were laid down beds of conglomerate, shale, sandstone and limestone, derived from the denudation of Archaean rocks, which, probably, rose as hills or mountains in parts of Peninsular India and along the Tibetan edge of the Himalayan region. These beds constitute the record of the long Purana Era[1] and are probably coeval with the Algonkian of North America. Even in these early times volcanic disturbances affected this area and the lower beds of the Purana deposits were penetrated by volcanic outflows, covered by sheets of lava, uplifted, denuded and again submerged

[1] See footnote, p. 139.

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beneath the waters. Two such periods of instability have left their records in the sediments of the Purana sea.

The succeeding era—the Dravidian Era—opens with Haimanta (Cambrian) times. A shallow sea now extended over Kumaun, Garwal, and Spiti, as well as Kashmir and ultimately over the Salt Range region of the Punjab as is shown by deposits in these areas. This sea was not, however, connected with the Cambrian sea of Europe. The fossil faunas left by the two seas are distinct.

After an interval of disturbance during closing Haimanta times, geographical changes attendant on further land movements occurred. The central sea of Asia, the Tethys, extended westwards and now joined with the European Paleozoic sea; and deposits of Ordovician and Silurian age were laid down:—the Muth deposits.

The succeeding Devonian Period saw the whole Northern Himalayan area under the waters of the Tethys which, eastward, extended to Burma and China and, westward, covered Kashmir, the Hindu Kush and part of Afghanistan. Deposits continued to be formed in this area till middle Carboniferous times.

Near. the close of the Dravidian Era Kashmir became convulsed by volcanic disturbance and the Penjal traps were ejected. It was a time of worldwide disturbance and of redistribution of land and water. Carboniferous times had begun, and the geographical changes in

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the southern limits of the Tethys are regarded as ushering in a new and last era in Indian geological history the Aryan Bra.

India was now part of Gondwanaland; that vanished continent which then reached westward to South Africa and eastward to Australia. A boulder-bed of glacial origin, the Talchir Boulder-bed, occurs in many surviving parts of this great land. It enters largely into the Salt Range deposits. There is evidence that extensive sheets of ice, wearing down the rocks of Rajputana, shoved their moraines northward into the Salt Range Sea; then, probably, a southern extension of the Tethys.

Subsequent to this ice age the Indian coalfields of the Gondwana were laid down, with beds rich in the Glossopteris and Gangamopteris flora. This remarkable carboniferous flora extends to Southern Kashmir, so that it is to be inferred that this region was also part of the main Gondwanaland. But its emergence was but for a brief period. Upper Carboniferous marine deposits succeeded; and, in fact, there was no important discontinuity in the deposits in this area from Panjal times till the early Tertiaries. During the whole of which vast period Kashmir was covered with the waters of the Tethys.

The closing Dravidian disturbances of the Kashmir region did not, apparently, extend to the eastern Himalayan area. But the Carboniferous Period was, in this

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eastern area, one of instability, culminating, at the close of the Period, in a steady rise of the land and a northward retreat of the Tethys. Nearly the entire Himalaya east of Kashmir became a land surface and remained exposed to denudative forces for so long a time that in places the whole of the Carboniferous, Devonian, and a large part of the Silurian and Ordovician deposits were removed—some thousands of feet in thickness—before resubmergence in the Tethys occurred.

Towards the end of the Palaeozoic Age the Aryan Tethys receded westwards, but still covered the Himalaya and was still connected with the European Palaeozoic sea. The Himalayan area (as well as Kashmir) remained submerged in its waters throughout the entire Mesozoic Age.

During Cretaceous times the Tethys became greatly extended, indicating a considerable subsidence of northwestern India, Afghanistan, Western Asia, and, probably, much of Tibet. The shallow-water character of the deposits of the Tibetan Himalaya indicates, however, a coast line near this region. Volcanic materials, now poured out, foreshadow the incoming of the great mountain-building epoch of the Tertiary Era. The enormous mass of the Deccan traps, possessing a volume which has been estimated at as much as 6,000 cubic miles, was probably extruded over the Northern Peninsular region during late Cretaceous times. The sea now began to retreat, and by the close of

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the Eocene, it had moved westward to Sind and Baluchistan. The movements of the Earth's crust were attended by intense volcanic activity, and great volumes of granite were injected into the sediments, followed by dykes and outflows of basic lavas.

The Tethys vanished to return no more. It survives in the Mediterranean of today. The mountain-building movements continued into Pliocene times. The Nummulite beds of the Eocene were, as the result, ultimately uplifted 18,500 feet over sea level, a total uplift of not less than 20,000 feet.

Thus with many vicissitudes, involving intervals of volcanic activity, local uplifting, and extensive local denudation, the Himalaya, which had originated in the sediments of the ancient Purana sea, far back in pre-Cambrian times, and which had developed potentially in a long sequence of deposits collecting almost continuously throughout the whole of geological time, finally took their place high in the heavens, where only the winds—faint at such altitudes—and the lights of heaven can visit their eternal snows.[1]

In this great history it is significant that the longest continuous series of sedimentary deposits which the world has known has become transfigured into the loftiest elevation upon its surface.

[1] See A Sketch of the Geography and Geology of the Himalaya Mountains and Tibet. By Colonel S. G. Burrard, R.E., F.R.S., and H. H. Hayden, F.G.S., Part IV. Calcutta, 1908.

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The diagrammatic sections of the Himalaya accompanying this brief description arc taken from the monograph of Burrard and Hayden (loc. cit.) on the Himalaya. Looking at the sections we see that some of the loftiest summits are sculptured in granite and other crystalline rocks. The appearance of these materials at the surface indicates the removal by denudation and the extreme metamorphism of much sedimentary deposit. The crystalline rocks, indeed, penetrate some of the oldest rocks in the world. They appear in contact with Archaean, Algonkian or early Palaeozoic rocks. A study of the sections reveals not only the severe earth movements, but also the immense amount of sedimentary deposits involved in the genesis of these alps. It will be noted that the vertical scale is not exaggerated relatively to the horizontal.[1] Although there is no evidence of mountain building

[1] To those unacquainted with the terminology of Indian geology the following list of approximate equivalents in time will be of use

Ngari Khorsum Beds - Pleistocene. Siwalik Series - Miocene and Pliocene. Sirmur Series - Oligocene. Kampa System - Eocene and Cretaceous. Lilang System - Triassic. Kuling System - Permian. Gondwana System - Carboniferous. Kenawar System - Carboniferous and Devonian Muth System - Silurian. Haimanta System - Mid. and Lower Cambrian. Purana Group - Algonkian. Vaikrita System - Archaean. Daling Series - Archaean.

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on a large scale in the Himalayan area till the Tertiary upheaval, it is, in the majority of cases, literally correct to speak of the mountains as having their generations like organic beings, and passing through all the stages of birth, life, death and reproduction. The Alps, the Jura, the Pyrenees, the Andes, have been remade more than once in the course of geological time, the debris of a worn-out range being again uplifted in succeeding ages.

Thus to dwell for a moment on one case only: that of the Pyrenees. The Pyrenees arose as a range of older Palmozoic rocks in Devonian times. These early mountains, however, were sufficiently worn out and depressed by Carboniferous times to receive the deposits of that age laid down on the up-turned edges of the older rocks. And to Carboniferous succeeded Permian, Triassic, Jurassic and Lower Cretaceous sediments all laid down in conformable sequence. There was then fresh disturbance and upheaval followed by denudation, and these mountains, in turn, became worn out and depressed beneath the ocean so that Upper Greensand rocks were laid down unconforrnably on all beneath. To these now succeeded Upper Chalk, sediments of Danian age, and so on, till Eocene times, when the tale was completed and the existing ranges rose from the sea. Today we find the folded Nummulitic strata of Eocene age uplifted 11,000 feet, or within 200 feet of the greatest heights of the Pyrenees. And so they stand awaiting

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the time when once again they shall "fall into the portion of outworn faces."[1]

Only mountains can beget mountains. Great accumulations of sediment are a necessary condition for the localisation of crust-flexure. The earliest mountains arose as purely igneous or volcanic elevations, but the generations of the hills soon originated in the collection of the debris, under the law of gravity, in the hollow places. And if a foundered range is exposed now to our view encumbered with thousands of feet of overlying sediments we know that while the one range was sinking, another, from which the sediments were derived, surely existed. Through the "windows" in the deep-cut rocks of the Swiss valleys we see the older Carboniferous Alps looking out, revisiting the sun light, after scores of millions of years of imprisonment. We know that just as surely as the Alps of today are founding by their muddy torrents ranges yet to arise, so other primeval Alps fed into the ocean the materials of these buried pre-Permian rocks.

This succession of events only can cease when the rocks have been sufficiently impoverished of the heat-producing substances, or the forces of compression shall have died out in the surface crust of the earth.

It seems impossible to escape the conclusion that in the great development of ocean-encircling areas of

[1] See Prestwich, Chemical and Physical Geology, p. 302.

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deposition and crustal folding, the heat of radioactivity has been a determining factor. We recognise in the movements of the sediments not only an influence localising and accelerating crustal movements, but one which, in subservience to the primal distribution of land and water, has determined some of the greatest geographical features of the globe.

It is no more than a step to show that bound up with the radioactive energy are most of the earthquake and volcanic phenomena of the earth. The association of earthquakes with the great geosynclines is well known. The work of De Montessus showed that over 94 per cent. of all recorded shocks lie in the geosynclinal belts. There can be no doubt that these manifestations of instability are the results of the local weakness and flexure which originated in the accumulation of energy denuded from the continents. Similarly we may view in volcanoes phenomena referable to the same fundamental cause. The volcano was, in fact, long regarded as more intimately connected with earthquakes than it, probably, actually is; the association being regarded in a causative light, whereas the connexion is more that of possessing a common origin. The girdle of volcanoes around the Pacific and the earthquake belt coincide. Again, the ancient and modern volcanoes and earthquakes of Europe are associated with the geosyncline of the greater Mediterranean, the Tethys of Mesozoic times. There is no difficulty in understanding in a

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general way the nature of the association. The earthquake is the manifestation of rupture and slip, and, as Suess has shown, the epicentres shift along that fault line where the crust has yielded.[1] The volcano marks the spot where the zone of fusion is brought so high in the fractured crust that the melted materials are poured out upon the surface.

In a recent work on the subject of earthquakes Professor Hobbs writes: "One of the most interesting of the generalisations which De Montessus has reached as a result of his protracted studies, is that the earthquake districts on the land correspond almost exactly to those belts upon the globe which were the almost continuous ocean basins of the long Secondary era of geological history. Within these belts the sedimentary formations of the crust were laid down in the greatest thickness, and the formations follow each other in relatively complete succession. For almost or quite the whole of this long era it is therefore clear that the ocean covered these zones. About them the formations are found interrupted, and the lacuna indicate that the sea invaded the area only to recede from it, and again at some later period to transgress upon it. For a long time, therefore, these earthquake belts were the sea basins—the geosynclines. They became later the rising mountains of the Tertiary period, and mountains they

[1] Suess, The Face of the Earth, vol. ii., chap. ii.

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are today. The earthquake belts are hence those portions of the earth's crust which in recent times have suffered the greatest movements in a vertical direction—they are the most mobile portions of the earth's crust."[1] Whether the movements attending mountain elevation and denudation are a connected and integral part of those wide geographical changes which result in submergence and elevation of large continental areas, is an obscure and complex question. We seem, indeed, according to the views of some authorities, hardly in a position to affirm with certainty that such widespread movements of the land have actually occurred, and that the phenomena are not the outcome of fluctuations of oceanic level; that our observations go no further than the recognition of positive and negative movements of the strand. However this may be, the greater part of mechanical denudation during geological time has been done on the mountain ranges. It is, in short, indisputable that the orogenic movements which uplift the hills have been at the basis of geological history. To them the great accumulations of sediments which now form so large a part of continental land are mainly due. There can be no doubt of the fact that these movements have swayed the entire history, both inorganic and organic, of the world in which we live.

[1] Hobbs, Earthquakes, p. 58.

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To sum the contents of this essay in the most general terms, we find that in the conception of denudation as producing the convection and accumulation of radiothermal energy the surface features of the globe receive a new significance. The heat of the earth is not internal only, but rather a heat-source exists at the surface, which, as we have seen, cannot prevail to the same degree within; and when the conditions become favourable for the aggregation of the energy, the crust, heated both from beneath and from above, assumes properties more akin to those of its earlier stages of development, the secular heat-loss being restored in the radioactive supplies. These causes of local mobility have been in operation, shifting somewhat from place to place, and defined geographically by the continental masses undergoing denudation, since the earliest times.

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ALPINE STRUCTURE

AN intelligent observer of the geological changes progressing in southern Europe in Eocene times would have seen little to inspire him with a premonition of the events then developing. The Nummulitic limestones were being laid down in that enlarged Mediterranean which at this period, save for a few islands, covered most of south Europe. Of these stratified remains, as well as of the great beds of Cretaceous, Jurassic, Triassic, and Permian sediments beneath, our hypothetical observer would probably have been regardless; just as today we observe, with an indifference born of our transitoriness, the deposits rapidly gathering wherever river discharge is distributing the sediments over the sea-floor, or the lime-secreting organisms are actively at work. And yet it took but a few millions of years to uplift the deposits of the ancient Tethys; pile high its sediments in fold upon fold in the Alps, the Carpathians, and the Himalayas; and—exposing them to the rigours of denudation at altitudes where glaciation, landslip, and torrent prevail—inaugurate a new epoch of sedimentation and upheaval.

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In the case of the Alps, to which we wish now specially to refer, the chief upheaval appears to have been in Oligocene times, although movement continued to the close of the Pliocene. There was thus a period of some millions of years within which the entire phenomena were comprised. Availing ourselves of Sollas' computations,[1] we may sum the maximum depths of sedimentary deposits of the geological periods concerned as follows:—

Pliocene - - - - - 3,950 m.

Miocene - - - - - 4,250 m.

Oligocene - - - - 3,660 m.

Eocene - - - - - - 6,100 m.

and assuming that the orogenic forces began their work in the last quarter of the Eocene period, we have a total of 13,400 m. as some measure of the time which elapsed. At the rate of io centimetres in a century these deposits could not have collected in less than 13.4 millions of years. It would appear that not less than some ten millions of years were consumed in the genesis of the Alps before constructive movements finally ceased.

The progress of the earth-movements was attended by the usual volcanic phenomena. The Oligocene and Miocene volcanoes extended in a band marked by the Auvergne, the Eiffel, the Bohemian, and the eastern Carpathian eruptions; and, later, towards the close of the movements in Pliocene times, the south border

[1] Sollas, Anniversary Address, Geol. Soc., London, 1909.

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regions of the Alps became the scene of eruptions such as those of Etna, Santorin, Somma (Vesuvius), etc.

We have referred to these well-known episodes with two objects in view: to recall to mind the time-interval involved, and the evidence of intense crustal disturbance, both dynamic and thermal. According to views explained in a previous essay, the energetic effects of radium in the sediments and upper crust were a principal factor in localising and bringing about these results. We propose now to inquire if, also, in the more intimate structure of the Alps, the radioactive energy may not have borne a part.

What we see today in the Alps is but a residue spared by denudation. It is certain that vast thicknesses of material have disappeared. Even while constructive effects were still in progress, denudative forces were not idle. Of this fact the shingle accumulations of the Molasse, where, on the northern borders of the Alps, they stand piled into mountains, bear eloquent testimony. In the sub-Apennine series of Italy, the great beds of clays, marls, and limestones afford evidence of these destructive processes continued into Pliocene times. We have already referred to Schmidt's estimate that the sedimentary covering must have in places amounted to from 15,000 to 20,000 metres. The evidence for this is mainly tectonic or structural; but is partly forthcoming in the changes which the materials now open to our inspection plainly reveal. Thus it is impos-

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sible to suppose that gneissic rocks can become so far plastic as to flow in and around the calcareous sediments, or be penetrated by the latter—as we see in the Jungfrau and elsewhere—unless great pressures and high temperatures prevailed. And, according to some writers, the temperatures revealed by the intimate structural changes of rock-forming minerals must have amounted to those of fusion. The existence of such conditions is supported by the observation that where the.crystallisation is now the most perfect, the phenomena of folding and injection are best developed.[1] These high temperatures would appear to be unaccountable without the intervention of radiothermal effects; and, indeed, have been regarded as enigmatic by observers of the phenomena in question. A covering of 20,000 metres in thickness would not occasion an earth-temperature exceeding 500 deg. C. if the gradients were such as obtain in mountain regions generally; and 600 deg. is about the limit we could ascribe to the purely passive effects of such a layer in elevating the geotherms.

Those who are still unacquainted with the recently published observations on the structure of the Alps may find it difficult to enter into what has now to be stated; for the facts are, indeed, very different from the generally preconceived ideas of mountain formation. Nor can we wonder that many geologists for long held

[1] Weinschenk, C. R. Congres Geol., 1900, p. 321, et seq.

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back from admitting views which appeared so extreme. Receptivity is the first virtue of the scientific mind; but, with every desire to lay aside prejudice, many felt unequal to the acceptance of structural features involving a folding of the earth-crust in laps which lay for scores of miles from country to country, and the carriage of mountainous materials from the south of the Alps to the north, leaving them finally as Alpine ranges of ancient sediments reposing on foundations of more recent date. The historian of the subject will have to relate how some who finally were most active in advancing the new views were at first opposed to them. In the change of conviction of these eminent geologists we have the strongest proof of the convincing nature of the observations and the reality of the tectonic features upon which the recent views are founded.

The lesser mountains which stand along the northern border of the great limestone Alps, those known as the Prealpes, present the strange characteristic of resting upon materials younger than themselves. Such mountains as the remarkable-looking Mythen, near Schwyz, for instance, are weathered from masses of Triassic and Jurassic rock, and repose on the much more recent Flysch. In sharp contrast to the Flysch scenery, they stand as abrupt and gigantic erratics, which have been transported from the central zone of the Alps lying far to the south. They are strangers petrologically,

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stratigraphically, and geographically,[1] to the locality in which they now occur. The exotic materials may be dolomites, limestones, schists, sandstones, or rocks of igneous origin. They show in every case traces of the severe dynamic actions to which they have been subjected in transit. The igneous, like the sedimentary, klippen, can be traced to distant sources; to the massif of Belladonne, to Mont Blanc, Lugano, and the Tyrol. The Prealpes are, in fact, mountains without local roots.

In this last-named essential feature, the Prealpes do not differ from the still greater limestone Alps which succeed them to the south. These giants, e.g. the Jungfrau, Wetterhorn, Eiger, etc., are also without local foundations. They have been formed from the overthrown and drawn-out anticlines of great crust-folds, whose synclines or roots are traceable to the south side of the Rhone Valley. The Bernese Oberland originated in the piling-up of four great sheets or recumbent folds, one of which is continued into the Prealpes. With Lugeon[2] we may see in the phenomenon of the formation of the Prealpes a detail; regarding it as a normal expression of that mechanism which has created the Swiss Alps. For these limestone masses of the Oberland are not indications of a merely local shift of the sedimentary covering of the Alps. Almost the whole covering has

[1] De Lapparent, Traite de Geologie, p. 1,785.

[2] Lugeon, Bulletin Soc. Geol. de France, 1901, p. 772.

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been pushed over and piled up to the north. Lugeon[l] concludes that, before denudation had done its work and cut off the Prealpes from their roots, there would have been found sheets, to the number of eight, superimposed and extending between the Mont Blanc massif and the massif of the Finsteraarhorn: these sheets being the overthrown folds of the wrinkled sedimentary covering. The general nature of the alpine structure

{Fig. 8}

will be understood from the presentation of it diagrammatically after Schmidt of Basel (Fig. 8).[2] The section extends from north to south, and brings out the relations of the several recumbent folds. We must imagine almost the whole of these superimposed folds now removed from the central regions of the Alps by denudation,

[1] Lugeon, loc. cit.

[2] Schmidt, Ec. Geol. Helvetiae, vol. ix., No. 4.

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and leaving the underlying gneisses rising through the remains of Permian, Triassic, and Jurassic sediments; while to the north the great limestone mountains and further north still, the Prealpes, carved from the remains of the recumbent folds, now stand with almost as little resemblance to the vanished mountains as the memories of the past have to its former intense reality.

These views as to the origin of the Alps, which are shared at the present day by so many distinguished geologists, had their origin in the labours of many now gone; dating back to Studer; finding their inspiration in the work of Heim, Suess, and Marcel Bertrand; and their consummation in that of Lugeon, Schardt, Rothpletz, Schmidt, and many others. Nor must it be forgotten that nearer home, somewhat similar phenomena, necessarily on a smaller scale, were recognised by Lapworth, twenty-six years ago, in his work on the structure of the Scottish Highlands.

An important tectonic principle underlies the development of the phenomena we have just been reviewing. The uppermost of the superimposed recumbent folds is more extended in its development than those which lie beneath. Passing downwards from the highest of the folds, they are found to be less and less extended both in the northerly and in the southerly direction, speaking of the special case—the Alps—now before us. This feature might be described somewhat differently. We might say that those folds which had their roots farther

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to the south were the most drawn-out towards the north: or again we might say that the synclinal or deep-seated part of the fold has lagged behind the anticlinal or what was originally the highest part of the fold, in the advance of the latter to the north. The anticline has advanced relatively to the syncline. To this law one exception only is observed in the Swiss Alps; the sheet of the Breche (Byecciendecke) falls short, in its northerly extension, of the underlying fold, which extends to form the Prealpes.

Contemplating such a generalised section as Professor Schmidt's, or, indeed, more particular sections, such as those in the Mont Blanc Massif by Marcel Bertrand,[1] of the Dent de Morcles, Diablerets, Wildhorn, and Massif de la Breche by Lugeon,[2] or finally Termier's section of the Pelvoux Massif,[3] one is reminded of the breaking of waves on a sloping beach. The wave, retarded at its base, is carried forward above by its momentum, and finally spreads far up on the strand; and if it could there remain, the succeeding wave must necessarily find itself superimposed upon the first. But no effects of inertia, no kinetic effects, may be called to our aid in explaining the formation of mountains. Some geologists have accordingly supposed that in order to account for

[1] Marcel Bertrand, Cong. Geol. Internat., 1900, Guide Geol., xiii. a, p. 41.

[2] Lugeon, loc. cit., p. 773.

[3] De Lapparent, Traite de Geol., p. 1,773.

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the recumbent folds and the peculiar phenomena of increasing overlap, or deferlement, an obstacle, fixed and deep-seated, must have arrested the roots or synclines of the folds, and held them against translational motion, while a movement of the upper crust drew out and carried forward the anticlines. Others have contented themselves by recording the facts without advancing any explanatory hypothesis beyond that embodied in the incontestable statement that such phenomena must be referred to the effects of tangential forces acting in the Earth's crust.

It would appear that the explanation of the phenomena of recumbent folds and their deferlement is to be obtained directly from the temperature conditions prevailing throughout the stressed pile of rocks; and here the subject of mountain tectonics touches that with which we were elsewhere specially concerned—the geological influence of accumulated radioactive energy.

As already shown[1], a rise of temperature due to this source of several hundred degrees might be added to such temperatures as would arise from the mere blanketing of the Earth, and the consequent upward movement of the geotherms. The time element is here the most important consideration. The whole sequence of events from the first orogenic movements to the final upheaval in Pliocene times must probably have occupied not less than ten million years.

[1] Mountain Genesis, p. 129, et seq.

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Unfortunately the full investigation of the distribution of temperature after any given time is beset with difficulties; the conditions being extremely complex. If the radioactive heating was strictly adiabatic—that is, if all the heat was conserved and none entered from without—the time required for the attainment of the equilibrium radioactive temperature would be just about six million years. The conditions are not, indeed, adiabatic; but, on the other hand, the rocks upraised by lateral pressure were by no means at 0 deg. C. to start with. They must be assumed to have possessed such temperatures as the prior radiothermal effects, and the conducted heat from the Earth's interior, may have established.

It would from this appear probable that if a duration of ten million years was involved, the equilibrium radioactive temperatures must nearly have been attained. The effects of heat conducted from the underlying earthcrust have to be added, leading to a further rise in temperature of not less than 500 deg. or 600 deg. . In such considerations the observed indications of high temperatures in materials now laid bare by denudation, probably find their explanation (P1. XIX).

The first fact that we infer from the former existence of such a temperature distribution is the improbability, indeed the impossibility, that anything resembling a rigid obstacle, or deep-seated "horst," can have existed beneath the present surface-level, and opposed the northerly movement of the deep-lying synclines. For

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such a horst can only have been constituted of some siliceous rock-material such as we find everywhere rising through the worn-down sediments of the Alps; and the idea that this could retain rigidity under the prevailing temperature conditions, must be dismissed. There is no need to labour this question; the horst cannot have existed. To what, then, is the retardation of the lower parts of the folds, their overthrow, above, to the north, and their deferlement, to be ascribed?

A little consideration shows that the very conditions of high temperature and viscosity, which render untenable the hypothesis of a rigid obstacle, suffice to afford a full explanation of the retardation of the roots of the folds. For directed translatory movements cannot be transmitted through a fluid, pressure in which is necessarily hydrostatic, and must be exerted equally in every direction. And this applies, not only to a fluid, but to a body which will yield viscously to an impressed force. There will be a gradation, according as viscosity gives place to rigidity, between the states in which the applied force resolves itself into a purely hydrostatic pressure, and in which it is transmitted through the material as a directed thrust. The nature of the force, in the most general case, of course, has to be considered; whether it is suddenly applied and of brief duration, or steady and long-continued. The latter conditions alone apply to the present case.

It follows from this that, although a tangential force

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or pressure be engendered by a crustal movement occurring to the south, and the resultant effects be transmitted northwards, these stresses can only mechanically affect the rigid parts of the crust into which they are carried. That is to say, they may result in folding and crushing, or horizontally transporting, the upper layers of the Earth's crust; but in the deeper-lying viscous materials they must be resolved into hydrostatic pressure which may act to upheave the overlying covering, but must refuse to transmit the horizontal translatory movements affecting the rigid materials above.

Between the regions in which these two opposing conditions prevail there will be no hard and fast line; but with the downward increase of fluidity there will be a gradual failure of the mechanical conditions and an increase of the hydrostatic. Thus while the uppermost layers of the crust may be transported to the full amount of the crustal displacement acting from the south (speaking still of the Alps) deeper down there will be a lesser horizontal movement, and still deeper there is no influence to urge the viscous rock-materials in a northerly direction. The consequences of these conditions must be the recumbence of the folds formed under the crust-stress, and their deferlement towards the north. To see this, we must follow the several stages of development.

The earliest movements, we may suppose, result in flexures of the Jura-Mountain type—that is, in a

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succession of undulations more or less symmetrical. As the orogenic force continues and develops, these undulations give place to folds, the limbs of which are approximately vertical, and the synclinal parts of which become ever more and more depressed into the deeper, and necessarily hotter, underlying materials; the anticlines being probably correspondingly elevated. These events are slowly developed, and the temperature beneath is steadily rising in consequence of the conducted interior heat, and the steady accumulation of radioactive energy in the sedimentary rocks and in the buried radioactive layer of the Earth. The work expended on the crushed and sheared rock also contributes to the developing temperature. Thus the geotherms must move upwards, and the viscous conditions extend from below; continually diminishing the downward range of the translatory movements progressing in the higher parts. While above the folded sediments are being carried northward, beneath they are becoming anchored in the growing viscosity of the medium. The anticlines will bend over, and the most southerly of the folds will gradually become pushed or bent over those lying to the north. Finally, the whole upper part of the sheaf will become horizontally recumbent; and as the uppermost folds will be those experiencing the greatest effects of the continued displacement, the deferlement or overlap must necessarily arise.

We may follow these stages of mountain evolution

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in a diagram (Fig. 9) in which we eliminate intermediate conditions, and regard the early and final stages of development only. In the upper sketch we suppose the lateral compression much developed and the upward movement of the geotherms in progress. The dotted line may be assumed to be a geotherm having a temperature of viscosity. If the conditions here shown persist

{Fig. 9}

indefinitely, there is no doubt that the only further developments possible are the continued crushing of the sediments and the bodily displacement of the whole mass to the north. The second figure is intended to show in what manner these results are evaded. The geotherm of viscosity has risen. All above it is affected mechanically by the continuing stress, and borne northwards in varying

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degree depending upon the rigidity. The folds have been overthrown and drawn out; those which lay originally most to the south have become the uppermost; and, experiencing the maximum amount of displacement, overlap those lying beneath. There has also been a certain amount of upthrow owing to the hydrostatic pressure. This last-mentioned element of the phenomena is of highly indeterminate character, for we know not the limits to which the hydrostatic pressure may be transmitted, and where it may most readily find relief. While, according to some of the published sections, the uplifting force would seem to have influenced the final results of the orogenic movements, a discussion of its effects would not be profitable.

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OTHER MINDS THAN OURS?

IN the year 1610 Galileo, looking through his telescope then newly perfected by his own hands, discovered that the planet Jupiter was attended by a train of tiny stars which went round and round him just as the moon goes round the Earth.

It was a revelation too great to be credited by mankind. It was opposed to the doctrine of the centrality of the Earth, for it suggested that other worlds constituted like ours might exist in the heavens.

Some said it was a mere optic illusion; others that he who looked through such a tube did it at the peril of his soul—it was but a delusion of Satan. Galileo converted a few of the unbelievers who had the courage to look through his telescope. To the others he said, he hoped they would see those moons on their way to heaven. Old as this story is it has never lost its pathos or its teaching.

The spirit which assailed Galileo's discoveries and which finally was potent to overshadow his declining years, closed in former days the mouths of those who asked the question written at the head of this lecture: "Are we to believe that there are other minds than ours?"

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Today we consider the question in a very different spirit. Few would regard it as either foolish or improper. Its intense interest would be admitted by all, and but for the limitations closing our way on every side it would, doubtless, attract the most earnest investigation. Even on the mere balance of judgment between the probable and the improbable, we have little to go on. We know nothing definitely as to the conditions under which life may originate: whether these are such as to be rare almost to impossibility, or common almost to certainty. Only within narrow limits of temperature and in presence of certain of the elements, can life like ours exist, and outside these conditions life, if such there be, must be different from ours. Once originated it is so constituted as to assail the energies around it and to advance from less to greater. Do we know more than these vague facts? Yes, we have in our experience one other fact and one involving much.

We know that our world is very old; that life has been for many millions of years upon it; and that Man as a thinking being is but of yesterday. Here is then a condition to be fulfilled. To every world is physically assigned a limit to the period during which it is habitable according to our knowledge of life and its necessities. This limit passed and rationality missed, the chance for that world is gone for ever, and other minds than ours assuredly will not from it contemplate the universe. Looking at our own world we see that the tree of life has,

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indeed, branched, leaved and, possibly, budded many times; it never bloomed but once.

All difficulties dissolve and speculations become needless under one condition only: that in which rationality may be inferred directly or indirectly by our observations on some sister world in space, This is just the evidence which in recent years has been claimed as derived from a study of the surface of Mars. To that planet our hope of such evidence is restricted. Our survey in all other directions is barred by insurmountable difficulties. Unless some meteoric record reached our Earth, revelationary of intelligence on a perished world, our only hope of obtaining such evidence rests on the observation of Mars' surface features. To this subject we confine our attention in what follows.

The observations made during recent years upon the surface features of Mars have, excusably enough, given rise to sensational reports. We must consider under what circumstances these observations have been made.

Mars comes into particularly favourable conditions for observation every fifteen years. It is true that every two years and two months we overtake him in his orbit and he is then in "opposition." That is, the Earth is between him and the sun: he is therefore in the opposite part of the heavens to the sun. Now Mars' orbit is very excentric, sometimes he is 139 million miles from the sun, and sometimes he as as much as 154 million miles from the sun. The Earth's orbit is, by comparison, almost

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a circle. Evidently if we pass him when he is nearest to the sun we see him at his best; not only because he is then nearest to us, but because he is then also most brightly lit. In such favourable oppositions we are within 35 million miles of him; if Mars was in aphelion we would pass him at a distance of 61 million miles. Opposition occurs under the most favourable circumstances every fifteen years. There was one in 1862, another in 1877, one in 1892, and so on.

When Mars is 35 million miles off and we apply a telescope magnifying 1,000 diameters, we see him as if placed 35,000 miles off. This would be seven times nearer than we see the moon with the naked eye. As Mars has a diameter about twice as great as that of the moon, at such a distance he would look fourteen times the diameter of the moon. Granting favourable conditions of atmosphere much should be seen.

But these are just the conditions of atmosphere of which most of the European observatories cannot boast. It is to the honour of Schiaparelli, of Milan, that under comparatively unfavourable conditions and with a small instrument, he so far outstripped his contemporaries in the observation of the features of Mars that those contemporaries received much of his early discoveries with scepticism. Light and dark outlines and patches on the planet's surface had indeed been mapped by others, and even a couple of the canals sighted; but at the opposition of 1877 Schiaparelli first mapped any considerable

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number of the celebrated "canals" and showed that these constituted an extraordinary and characteristic feature of the planet's geography. He called them "canali," meaning thereby "channels." It is remarkable indeed that a mistranslation appears really responsible for the initiation of the idea that these features are canals.

In 1882 Schiaparelli startled the astronomical world by declaring that he saw some of the canals double—that is appearing as two parallel lines. As these lines span the planet's surface for distances of many thousands of miles the announcement naturally gave rise to much surprise and, as I have said, to much scepticism. But he resolutely stuck to his statement. Here is his map of 1882. It is sufficiently startling.

In 1892 he drew a new map. It adds a little to the former map, but the doubling was not so well seen. It is just the strangest feature about this doubling that at times it is conspicuous, at times invisible. A line which is distinctly seen as a single line at one time, a few weeks later will appear distinctly to consist of two parallel lines; like railway tracks, but tracks perhaps 200 miles apart and up to 3,000 or even 4,000 miles in length.

Many speculations were, of course, made to account for the origin of such features. No known surface peculiarity on the Earth or moon at all resembles these features. The moon's surface as you know is cracked and

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streaked. But the cracks are what we generally find cracks to be—either aimless, wandering lines, or, if radiating from a centre, then lines which contract in width as they leave the point of rupture. Where will we find cracks accurately parallel to one another sweeping round a planet's face with steady curvature for, 4,000 miles, and crossing each other as if quite unhampered by one another's presence? If the phenomenon on Mars be due to cracks they imply a uniformity in thickness and strength of crust, a homogeneity, quite beyond all anticipation. We will afterwards see that the course of the lines is itself further opposed to the theory that haphazard cracking of the crust of the planet is responsible for the lines. It was also suggested that the surface of the planet was covered with ice and that these were cracks in the ice. This theory has even greater difficulties than the last to contend with. Rivers have been suggested. A glance at our own maps at once disposes of this hypothesis. Rivers wander just as cracks do and parallel rivers like parallel cracks are unknown.

In time the many suggestions were put aside. One only remained. That the lines are actually the work of intelligence; actually are canals, artificially made, constructed for irrigation purposes on a scale of which we can hardly form any conception based on our own earthly engineering structures.

During the opposition of 1894, Percival Lowell, along with A. E. Douglass, and W. H. Pickering,

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observed the planet from the summit of a mountain in Arizona, using an 18-inch refracting telescope and every resource of delicate measurement and spectroscopy. So superb a climate favoured them that for ten months the planet was kept under continual observation. Over 900 drawings were made and not only were Schiaparelli's channels confirmed, but they added 116 to his 79, on that portion of the planet visible at that opposition. They made the further important discovery that the lines do not stop short at the dark regions of the planet's surface, as hitherto believed, but go right on in many cases; the curvature of the lines being unaltered.

Lowell is an uncompromising advocate of the "canal" theory. If his arguments are correct we have at once an answer to our question, "Are there other minds than ours?"

We must consider a moment Lowell's arguments; not that it is my intention to combat them. You must form your own conclusions. I shall lay before you another and, as I venture to think, more adequate hypothesis in explanation of the channels of Schiaparelli. We learn, however, much from Lowell's book—it is full of interest.[1]

Lowell lays a deep foundation. He begins by showing that Mars has an atmosphere. This must be granted him till some counter observations are made.

[1] Mars, by Percival Lowell (Longmans, Green & Co.), 1896,

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It is generally accepted. What that atmosphere is, is another matter. He certainly has made out a good case for the presence of water as one of its constituents,

It was long known that Mars possessed white regions at his poles, just as our Earth does. The waning of these polar snows—if indeed they are such—with the advance of the Martian summer, had often been observed. Lowell plots day by day this waning. It is evident from his observations that the snowfall must be light indeed. We see in his map the south pole turned towards us. Mars in perihelion always turns his south pole towards the sun and therefore towards the Earth. We see that between the dates June 3rd to August 3rd—or in two months—the polar snow had almost completely vanished. This denotes a very scanty covering. It must be remembered that Mars even when nearest to the sun receives but half our supply of solar heat and light.

But other evidence exists to show that Mars probably possesses but little water upon his surface. The dark places are not water-covered, although they have been named as if they were, indeed, seas and lakes. Various phenomena show this. The canals show it. It would never do to imagine canals crossing the seas. No great rivers are visible. There is a striking absence of clouds. The atmosphere of Mars seems as serene as that of Venus appears to be cloudy. Mists and clouds, however, sometime appear to veil his face and add to the difficulty of

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making observations near the limb of the planet. Lowell concludes it must be a calm and serene atmosphere; probably only one-seventh of our own in density. The normal height of the barometer in Mars would then be but four and a half inches. This is a pressure far less than exists on the top of the highest terrestrial mountain. A mountain here must have an altitude of about ten miles to possess so low a pressure on its summit. Drops of water big enough to form rain can hardly collect in such a rarefied atmosphere. Moisture will fall as dew or frost upon the ground. The days will be hot owing to the unimpeded solar radiation; the nights bitterly cold owing to the free radiation into space.

We may add that in such a climate the frost will descend principally upon the high ground at night time and in the advancing day it will melt. The freer radiation brings about this phenomenon among our own mountains in clear and calm weather.

With the progressive melting of the snow upon the pole Lowell connected many phenomena upon the planet's surface of much interest. The dark spaces appear to grow darker and more greenish. The canals begin to show themselves and reveal their double nature. All this suggests that the moisture liberated by the melting of the polar snow with the advancing year, is carrying vitality and springtime over the surface of the planet. But how is the water conveyed?

Lowell believes principally by the canals. These are

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constructed triangulating the surface of the planet in all directions. What we see, according to Lowell, is not the canal itself, but the broad band of vegetation which springs up on the arrival of the water. This band is perhaps thirty or forty miles wide, but perhaps much less, for Lowell reports that the better the conditions of observation the finer the lines appeared, so that they may be as narrow, possibly, as fifteen miles. It is to be remarked that a just visible dot on the surface of Mars must possess a diameter of 30 miles. But a chain of much smaller dots will be visible, just as we can see such fine objects as spiders' webs. The widening of the canals is then accounted for, according to Lowell, by the growth of a band of vegetation, similar to that which springs into existence when the floods of the Nile irrigate the plains of Egypt.

If no other explanation of the lines is forthcoming than that they are the work of intelligence, all this must be remembered. If all other theories fail us, much must be granted Lowell. We must not reason like fishes—as Lowell puts it—and deny that intelligent beings can thrive in an atmospheric pressure of four and half inches of mercury. Zurbriggen has recently got to the top of Aconcagua, a height of 24,000 feet. On the summit of such a mountain the barometer must stand at about ten inches. Why should not beings be developed by evolution with a lung capacity capable of living at two and a half times this altitude. Those steadily

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curved parallel lines are, indeed, very unlike anything we have experience of. It would be rather to be expected that another civilisation than our own would present many wide differences in its development.

What then is the picture we have before us according to Lowell? It is a sufficiently dramatic one.

Mars is a world whose water supply, never probably very abundant, has through countless years been drying up, sinking into his surface. But the inhabitants are making a brave fight for it, They have constructed canals right round their world so that the water, which otherwise would run to waste over the vast deserts, is led from oasis to oasis. Here the great centres of civilisation are placed: their Londons, Viennas, New Yorks. These gigantic works are the works of despair. A great and civilised world finds death staring it in the face. They have had to triple their canals so that when the central canal has done its work the water is turned into the side canals, in order to utilise it as far as possible. Through their splendid telescopes they must view our seas and ample rivers; and must die like travellers in the desert seeing in a mirage the cool waters of a distant lake.

Perhaps that lonely signal reported to have been seen in the twilight limb of Mars was the outcome of pride in their splendid and perishing civilisation. They would leave some memory of it: they would have us witness how great was that civilisation before they perish!

I close this dramatic picture with the poor comfort

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that several philanthropic people have suggested signalling to them as a mark of sympathy. It is said that a fortune was bequeathed to the French Academy for the purpose of communicating with the Martians. It has been suggested that we could flash signals to them by means of gigantic mirrors reflecting the light of our Sun. Or, again, that we might light bonfires on a sufficiently large scale. They would have to be about ten miles in diameter! A writer in the Pall Mall Gazette suggested that there need really be no difficulty in the matter. With the kind cooperation of the London Gas Companies (this was before the days of electric lighting) a signal might be sent without any additional expense if the gas companies would consent to simultaneously turn off the gas at intervals of five minutes over the whole of London, a signal which would be visible to the astronomers in Mars would result. He adds, naively: "If only tried for an hour each night some results might be obtained."

II

We have reviewed the theory of the artificial construction of the Martian lines. The amount of consideration we are disposed to give to the supposition that there are upon Mars other minds than ours will—as I have stated—necessarily depend upon whether or not we can assign a probable explanation of the lines upon purely physical grounds. If it is apparent that such

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lines would be formed with great probability under certain conditions, which conditions are themselves probable, then the argument by exclusion for the existence of civilisation on Mars, at once breaks down.

{Fig. 10}

As a romance writer is sometimes under the necessity of transporting his readers to other scenes, so I must now ask you to consent to be transported some millions

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of miles into the region of the heavens which lies outside Mars' orbit.

Between Mars and Jupiter is a chasm of 341 millions of miles. This gap in the sequence of planets was long known to be quite out of keeping with the orderly succession of worlds outward from the Sun. A society was formed at the close of the last century for the detection of the missing world. On the first day of the last century, Piazzi—who, by the way, was not a member of the society—discovered a tiny world in the vacant gap. Although eagerly welcomed, as better than nothing, it was a disappointing find. The new world was a mere rock. A speck of about 160 miles in diameter. It was obviously never intended that such a body should have all this space to itself. And, sure enough, shortly after, another small world was discovered. Then another was found, and another, and so on; and now more than 400 of these strange little worlds are known.

But whence came such bodies? The generally accepted belief is that these really represent a misbegotten world. When the Sun was younger he shed off the several worlds of our system as so many rings. Each ring then coalesced into a world. Neptune being the first born; Mercury the youngest born.

After Jupiter was thrown off, and the Sun had shrunk away inwards some 20o million miles, he shed off another ring. Meaning that this offspring of his should grow up like the rest, develop into a stable world with the

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potentiality even, it may be, of becoming the abode of rational beings. But something went wrong. It broke up into a ring of little bodies, circulating around him.

It is probable on this hypothesis that the number we are acquainted with does not nearly represent the actual number of past and present asteroids. It would take 125,000 of the biggest of them to make up a globe as big as our world. They, so far as they are known, vary in size from 10 miles to 160 miles in diameter. It is probable then—on the assumption that this failure of a world was intended to be about the mass of our Earth—that they numbered, and possibly number, many hundreds of thousands.

Some of these little bodies are very peculiar in respect to the orbits they move in. This peculiarity is sometimes in the eccentricity of their orbits, sometimes in the manner in which their orbits are tilted to the general plane of the ecliptic, in which all the other planets move.

The eccentricity, according to Proctor, in some cases may attain such extremes as to bring the little world inside Mars' mean distance from the sun. This, as you will remember, is very much less than his greatest distance from the sun. The entire belt of asteroids—as known—lie much nearer to Mars than to Jupiter.

As regards the tilt of their orbits, some are actually as much as 34 degrees inclined to the ecliptic, so that in fact they are seen from the Earth among our polar constellations.

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From all this you see that Mars occupies a rather hot comer in the solar system. Is it not possible that more than once in the remote past Mars may have encountered one of these wanderers? If he came within a certain distance of the small body his great mass would sway it from its orbit, and under certain conditions he would pick up a satellite in this manner. That his present satellites were actually so acquired is the suggestion of Newton, of Yale College.

Mars' satellites are indeed suspiciously and most abnormally small. I have not time to prove this to you by comparison with the other worlds of the solar system. In fact, they were not discovered till 1877—although they were predicted in a most curious manner, with the most uncannily accurate details, by Swift.

One of these bodies is about 36 miles in diameter. This is Phobos. Phobos is only 3.700 miles from the surface of Mars. The other is smaller and further off. He is named Deimos, and his diameter is only 10 miles. He is 12,500 miles from Mars' surface. With the exception of Phobos the next smallest satellite known in the solar system is one of Saturn's—Hyperion; almost 800 miles in diameter. The inner one goes all round Mars in 71/2 hours. This is Phobos' month. Mars turns on his axis in 24 hours and 40 minutes, so that people in Mars would see the rise of Phobos twice in the course of a day and night; lie would apparently cross the sky

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going against the other satellite; that is, he would move apparently from west to east.

We may at least assume as probable that other satellites have been gathered by Mars in the past from the army of asteroids.

Some of the satellites so picked up would be direct: that is, would move round the planet in the direction of his axial rotation. Others, on the chances, would be retrograde: that is, would move against his axial rotation. They would describe orbits making the same various angles with the ecliptic as do the asteroids; and we may be sure they would be of the same varying dimensions.

We go on to inquire what would be the consequence to Mars of such captures.

A satellite captured in this manner is very likely to be pulled into the Planet. This is a probable end of a satellite in any case. It will probably be the end of our satellite too. The satellite Phobos is indeed believed to be about to take this very plunge into his planet. But in the case when the satellite picked up happens to be rotating round the planet in the opposite direction to the axial rotation of the planet, it is pretty certain that its career as a satellite will be a brief one. The reasons for this I cannot now give. If, then, Mars picked up satellites he is very sure to have absorbed them sooner or later. Sooner if they happened to be retrograde satellites, later if direct satellites. His present satellites are recent additions. They are direct.

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The path of an expiring satellite will be a slow spiral described round the planet. The spiral will at last, after many years, bring the satellite down upon the surface of the primary. Its final approach will be accelerated if the planet possesses an atmosphere, as Mars probably does. A satellite of the dimensions of Phobos—that is 36 miles in diameter—would hardly survive more than 30 to 60 years within seventy miles of Mars' surface. It will then be rotating round Mars in an hour and forty minutes, moving, in fact, at the rate of 2.2 miles per second. In the course of this 30 or 60 years it will, therefore, get round perhaps 200,000 times, before it finally crashes down upon the Martians. During this closing history of the satellite there is reason to believe, however, that it would by no means pursue continually the same path over the surface of the planet. There are many disturbing factors to be considered. Being so small any large surface features of Mars would probably act to perturb the orbit of the satellite.

The explanation of Mars' lines which I suggest, is that they were formed by the approach of such satellites in former times. I do not mean that they are lines cut into his surface by the actual infall of a satellite. The final end of the satellite would be too rapid for this, I think. But I hope to be able to show you that there is reason to believe that the mere passage of the satellite, say at 70 miles above the surface of the planet, will, in itself, give rise to effects on the crust of the planet capable

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of accounting for just such single or parallel lines as we see.

In the first place we have to consider the stability of the satellite. Even in the case of a small satellite we cannot overlook the fact that the half of the satellite near the planet is pulled towards the planet by a gravitational force greater than that attracting the outer half, and that the centrifugal force is less on the inner than on the outer hemisphere. Hence there exists a force tending to tear the satellite asunder on the equatorial section tangential

{Fig. 11}

to the planet's surface. If in a fluid or plastic state, Phobos, for instance, could not possibly exist near the planet's surface. The forces referred to would decide its fate. It may be shown by calculation, however, that if Phobos has the strength of basalt or glass there would remain a considerable coefficient of safety in favour of the satellite's stability; even when the surfaces of planet and satellite were separated by only five miles.

We have now to consider some things which we expect will happen before the satellite takes its final plunge into the planet.

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This diagram (Fig. 11) shows you the satellite travelling above the surface of the planet. The satellite is advancing towards, or away from, the spectator. The planet is supposed to show its solid crust in cross section, which may be a few miles in thickness. Below this is such a hot plastic magma as we have reason to believe underlies much of the solid crust of our own Earth. Now there is an attraction between the satellite and the crust of the planet; the same gravitational attraction which exists between every particle of matter in the universe. Let us consider how this attraction will affect the planet's crust. I have drawn little arrows to show how we may consider the attraction of the satellite pulling the crust of the planet not only upwards, but also pulling it inwards beneath the satellite. I have made these arrows longer where calculation shows the stress is greater. You see that the greatest lifting stress is just beneath the satellite, whereas the greatest stress pulling the crust in under the satellite is at a point which lies out from under the satellite, at a considerable distance. At each side of the satellite there is a point where the stress pulling on the crust is the greatest. Of the two stresses the lifting stress will tend to raise the crust a little; the pulling stress may in certain cases actually tear the crust across; as at A and B.

It is possible to calculate the amount of the stress at the point at each side of the satellite where the stress is at its greatest. We must assume the satellite to be a certain size and density; we must also assume the crust of

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Mars to be of some certain density. To fix our ideas on these points I take the case of the present satellite Phobos. What amount of stress will he exert upon the crust of Mars when he approaches within, say, 40 miles of the planet's surface? We know his size approximately—he is about 36 miles in diameter. We can guess his density to be between four times that of water and eight times that of water. We may assume the density of Mars' surface to be about the same as that of our Earth's surface, that is three times as dense as water. We now find that the greatest stress tending to rend open the surface crust of Mars will be between 4,000 and 8,000 pounds to the square foot according to the density we assign to Phobos.

Will such a stress actually tear open the crust? We are not able to answer this question with any certainty. Much will depend upon the nature and condition of the crust. Thus, suppose that we are here (Fig. 12) looking down upon the satellite which is moving along slowly relatively to Mars' surface, in the direction of the arrow. The satellite has just passed over a weak and cracked part of the planet's crust. Here the stress has been sufficient to start two cracks. Now you know how easy it is to tear a piece of cloth when you go to the edge of it in order to make a beginning. Here the stress from the satellite has got to the edge of the crust. It is greatly concentrated just at the extremities of the cracks. It will, unler such circumstances probably carry on the

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tear. If it does not do so this time, remember the satellite will some hours later be coming over the same place again, and then again for, at least, many hundreds of times. Then also we are not limited to the assumption that the

{Fig. 12}

satellite is as small as Phobos. Suppose we consider the case of a satellite approaching Mars which has a diameter double that of Phobos; a diameter still much less than that of the larger class of asteroids. Even at the distance

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of 65 miles the stress will now amount to as much as from 15 to 30 tons per square foot. It is almost certain that such a stress repeated a comparatively few times over the same parts of the planet's surface would so rend the crust as to set up lines along which plutonic action would find a vent. That is, we might expect along these lines all the phenomena of upheaval and volcanic eruption which give rise to surface elevations.

The probable effect of a satellite of this dimension travelling slowly relatively to the surface of Mars is, then, to leave a very conspicuous memorial of his presence behind him. You see from the diagram that this memorial will consist o: two parallel lines of disturbance.

The linear character of the gravitational effects of the satellite is due entirely to the motion of the satellite relatively to the surface of the planet. If the satellite stood still above the surface the gravitational stress in the crust would, of course, be exerted radially outwards from the centre of the satellite. It would attain at the central point beneath the satellite its maximum vertical effect, and at some radial distance measured outwards from this point, which distance we can calculate, its maximum horizontal tearing effect. When the satellite moves relatively to the planet's crust, the horizontal tearing force acts differently according to whether it is directed in the line of motion or at right angles to this line.