The Birth-Time of the World and Other Scientific Essays Part 14

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 Gondwa.n.a.land. 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 ma.s.s 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 d.y.k.es 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 alt.i.tudes--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 - Tria.s.sic.

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 pa.s.sing 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, Tria.s.sic, Jura.s.sic 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 acc.u.mulations 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 enc.u.mbered 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 a.s.sociation 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 acc.u.mulation 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 a.s.sociation 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 a.s.sociated 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 a.s.sociation. The earthquake is the manifestation of rupture and slip, and, as Suess has shown, the epicentres s.h.i.+ft 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 acc.u.mulations 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 acc.u.mulation 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, a.s.sumes 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, s.h.i.+fting somewhat from place to place, and defined geographically by the continental ma.s.ses 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, Jura.s.sic, Tria.s.sic, 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 alt.i.tudes 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.