Mountain Genesis

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Title:     Mountain Genesis
Author: John Joly [More Titles by Joly]

OUR ancestors regarded mountainous regions with feelings of horror, mingled with commiseration for those whom an unkindly destiny had condemned to dwell therein. We, on the other hand, find in the contemplation of the great alps of the Earth such peaceful and elevated thoughts, and such rest to our souls, that it is to those very solitudes we turn to heal the wounds of ife. It is difficult to explain the cause of this very different point of view. It is probably, in part, to be referred to that cloud of superstitious horror which, throughout the Middle Ages, peopled the solitudes with unknown terrors; and, in part, to the asceticism which led the pious to regard the beauty and joy of life as snares to the soul's well-being. In those eternal solitudes where the overwhelming forces of Nature are most in evidence, an evil principle must dwell or a dragon's dreadful brood must find a home.

But while in our time the aesthetic aspect of the hills appeals to all, there remains in the physical history of the mountains much that is lost to those who have not shared in the scientific studies of alpine structure and genesis. They lose a past history which for interest competes with anything science has to tell of the changes of the Earth.

Great as are the physical features of the mountains compared with the works of Man, and great as are the forces involved compared with those we can originate or control, the loftiest ranges are small contrasted with the dimensions of the Earth. It is well to bear this in mind. I give here (Pl. XV.) a measured drawing showing a sector cut from a sphere of 50 cms. radius; so much of it as to exhibit the convergence of its radial boundaries which if prolonged will meet at the centre. On the same scale as the radius the diagram shows the highest mountains and the deepest ocean. The average height of the land and the average depth of the ocean are also exhibited. We see how small a movement of the crust the loftiest elevation of the Himalaya represents and what a little depression holds the ocean.

Nevertheless, it is not by any means easy to explain the genesis of those small elevations and depressions. It would lead us far from our immediate subject to discuss the various theoretical views which have been advanced to account for the facts. The idea that mountain folds, and the lesser rugosities of the Earth's surface, arose in a wrinkling of the crust under the influence of cooling and skrinkage of the subcrustal materials, is held by many eminent geologists, but not without dissent from others.

The most striking observational fact connected with mountain structure is that, without exception, the ranges of the Earth are built essentially of sedimentary rocks: that is of rocks which have been accumulated at some remote past time beneath the surface of the ocean. A volcanic core there may sometimes be--probably an attendant or consequence of the uplifting--or a core of plutonic igneous rocks which has arisen under the same compressive forces which have bowed and arched the strata from their original horizontal position. It is not uncommon to meet among unobservant people those who regard all mountain ranges as volcanic in origin. Volcanoes, however, do not build mountain ranges. They break out as more or less isolated cones or hills. Compare the map of the Auvergne with that of Switzerland; the volcanoes of South Italy with the Apennines. Such great ranges as those which border with triple walls the west coast of North America are in no sense volcanic: nor are the Pyrenees, the Caucasus, or the Himalaya. Volcanic materials are poured out from the summits of the Andes, but the range itself is built up of folded sediments on the same architecture as the other great ranges of the Earth.

Before attempting an explanation of the origin of the mountains we must first become more closely acquainted with the phenomena attending mountain elevation.

At the present day great accumulations of sediment are taking place along the margins of the continents where the rivers reach the ocean. Thus, the Gulf of Mexico receiving the sediment of the Mississippi and Rio Grande; the northeast coast of South America receiving the sediments of the Amazons; the east coast of Asia receiving the detritus of the Chinese rivers; are instances of such areas of deposition. Year by year, century by century, the accumulation progresses, and as it grows the floor of the sea sinks under the load. Of the yielding of the crust under the burthen of the sediments we are assured; for otherwise the many miles of vertically piled strata which are uplifted to our view in the mountains, never could have been deposited in the coastal seas of the past. The flexure and sinking of the crust are undeniable realities.

Such vast subsiding areas are known as geosynclines. From the accumulated sediments of the geosynclines the mountain ranges of the past have in every case originated; and the mountains of the future will assuredly arise and lofty ranges will stand where now the ocean waters close over the collecting sediments. Every mountain range upon the Earth enforces the certainty of this prediction.

The mountain-forming movement takes place after a certain great depth of sediment is collected. It is most intense where the thickness of deposit is greatest. We see this when we examine the structure of our existing mountain ranges. At either side where the sediments thin out, the disturbance dies away, till we find the comparatively shallow and undisturbed level sediments which clothe the continental surface.

Whatever be the connection between the deposition and the subsequent upheaval, _the element of great depth of accumulation seems a necessary condition and must evidently enter as a factor into the Physical Processes involved_. The mountain range can only arise where the geosyncline is deeply filled by long ages of sedimentation.

Dana's description of the events attending mountain building is impressive:

"A mountain range of the common type, like that to which the Appalachians belong, is made out of the sedimentary formations of a long preceding era; beds that were laid down conformably, and in succession, until they had reached the needed thickness; beds spreading over a region tens of thousands of square miles in area. The region over which sedimentary formations were in progress in order to make, finally, the Appalachian range, reached from New York to Alabama, and had a breadth of 100 to 200 miles, and the pile of horizontal beds along the middle was 40,000 feet in depth. The pile for the Wahsatch Mountains was 60,000 feet thick, according to King. The beds for the Appalachians were not laid down in a deep ocean, but in shallow waters, where a gradual subsidence was in progress; and they at last, when ready for the genesis, lay in a trough 40,000 feet deep, filling the trough to the brim. It thus appears that epochs of mountain-making have occurred only after long intervals of quiet in the history of a continent."

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 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 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 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 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 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 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.

[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.

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.

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 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. 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 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 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 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.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.

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.

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.

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 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 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 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 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 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 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.

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.

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.

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

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 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 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.

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." 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.

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.

[The end]
John Joly's Essay: Mountain Genesis

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