A SKETCH OF THE GEOLOGY OF INDIA

Sir EDWIN PASCOE

THREE Geological Regions.—Geologically India may be divided into three regions : (i) the Peninsula ; (ii) the Extra-Peninsular region, including Baluchistan, the North-West Frontier, the portion of the Punjab north-west of the Jhelum including the Salt Range, the Himalaya, Burma and the Andaman and Nicobar islands ; and (iii) the Indo-Gangetic Alluvial Plain, between the first two. The Shillong Plateau belongs to the Peninsular region.

Antiquity of Deposits and Physiography of the Peninsula.--The keynote of the history of the peninsula is immeasurable antiquity- antiquity even according to geological standards. This applies emphatically to the formation of the rocks themselves, but also in no small measure to their elevation above the sea to form land. Of the rocks, omitting a few small but highly interesting coast deposits, omitting the coal basins, and omitting a vast lava flood which poured over the older rocks, probably none is younger than the Cambrian, the earliest geological period in which organic remains are definitely recognizable. By far the greater bulk of the Peninsular rocks, however, date back to still older periods which have left no record of life upon the globe. Soon after the Cambrian period practically the whole of the Peninsular region was raised to form part of a continental area. Land it became and land it has been ever since.

Dharwar System,—The oldest recognizable rocks are Archaean in age and have been assigned the name of Dharwar after the district in the Bombay Presidency where they were first studied. The Dharwar rocks include true sediments and lava flows, and these, of course, must have been deposited upon some floor. For many years it was thought that most of the gneiss, which covers such a large proportion of the Peninsula, represented this most ancient ocean floor upon which the earliest sedimentary deposits of India were laid down. During the past few years, however, it has been shown that much of the gneiss is the altered product of a molten magma which was intruded into the Dharwar sediments after they had been deposited; this gneiss, therefore, though of great age, must be looked upon as younger than the Dharwar strata. So much of the gneiss has been shown to be intrusive into the Dharwar that it is now impossible to point to any of it as being definitely a remnant of the primeval ocean floor of the Dharwar period—the floor which received the first sediments, of which any record remains, brought down by the rivers from that very early land.

Primeval Ocean Floor.—Nevertheless, an ocean floor there must have been or the Dharwar sediments could never have been deposited, and it seems unlikely that the junction between the sediments and the floor has everywhere been completely obliterated by the subsequent intrusion of molten rock. In places the lowest horizon of the Dharwars in contact with the gneiss is a conglomerate of what appear to be pebbles of the gneiss, and this was originally regarded as conclusive proof that the gneiss was the older of the two and had supplied pebbles to the Dharwar rivers. Most of these conglomerates are now regarded as subsequent to the induration and folding of the rocks and produced by fracture and relative movement, the so-called " pebbles " being merely fragments detached and ground into more or less globular shape by the movement of the two uneven walls of the fissure against one another.

Himalayan Gneiss.—Great thicknesses of gneiss of varying character form the cores of the Himalayan ranges but here, owing to the great intensity of a vastly younger N.-S. earth movement—a movement which in fact may be still proceeding—it is even more difficult to decide whether any of it is metamorphosed Dharwar sediment, or whether any of it belongs to the primeval ocean floor of igneous rocks on which the first sediments were deposited. Some of it is obviously younger than either and can be seen to have intruded itself in the form of molten granite magma into older rocks. The Chor massif near Simla is an example and is thought to have been intruded into the Jutogh rocks which are possibly the local representatives of the Archaean, during the earlier half of the Purana period. Some of the Himalayan gneisses, again, are merely metamorphosed Palaeozoic sediments. Considering the great size of the Himalayan uplift, it seems highly probable that the Dharwars, if they ever extended so far north, or the igneous crust which in the peninsula formed the floor on which they rested, or both, have been forced up to high altitudes and laid bare by the intensive denudation which followed. Himalayan geology is in its infancy, bat systematic geological mapping on large scale maps will no doubt in time unravel most of the complications which metamorphism and movement have produced, and decide whether this ancient system is present in the heart of this giant uplift.

Mergui Series.—In the Tenasserim division of Burma are some ancient metamorphosed clays and volcanic material which are regarded as the probable equivalents of the Dharwar system. Amongst the fragments of a volcanic agglomerate were found a few rounded pieces of a granite which has not, so far, been observed in situ. These are interesting as being representatives of a rock older than the deposits in which they occur ; if the correlation of this " Mergui series " with the Dharwar be correct, this granite is the oldest rock we definitely know of in the Indian Empire.

Dharwar Sea.—Of the extent of the Dharwar Sea we have very little knowledge ; it covered the northern and probably also the southern half of Madras and stretched northwards at least as far as the alluvial belt. Whether it covered any portion of the Himalayan region or beyond we cannot at present say. It probably covered the Shillong plateau and extended as far east as Burma. It is from the Dharwar beds that the Kolar and other great goldfields of Mysore and Madras derive their gold. Manganese, iron and copper are also valuable products from this very ancient system.

Folding Movement.—Whatever the mutual relationship between the Dharwars and the gneiss may have been, we know that the two were afterwards folded up tightly together by a compressional movement acting in a more or less E.-W. direction. The two groups were thus thrown into sharp folds running generally N.-S. but veering to N.W.-S.E. This folding movement was accompanied or followed by upheaval and the formation of land. Of the shape and extent of this land area—this forerunner of India—we know scarcely anything ; the tightness and frequency of the folds, together with the enormous quantity of rock which must have been stripped off by subsequent denudation, point to lofty mountainous country which may have stretched far beyond the confines of the present peninsula. Should the Archaean, including the Dharwars, be represented by any of the Himalayan gneiss, it would of course be difficult to recognize therein any of these N.-S. folds, owing to the severity of the younger movement referred to above, which long afterwards produced more or less E.-W. folds and would have largely obliterated any older N.-S. folds. Nevertheless any old N.-S. folds in the Himalayan region would have affected the result of the later movement, and it is not impossible that some of the transverse strikes, faulting and orientation of outcrops amongst the Himalayan rocks may owe their existence partly to the effect of these older folds. After the E.-W. movement an immense period of quiescence ensued, during which the continent was subjected to atmospheric denudation so prolonged as to wear off almost all the old Dharwar deposits. The decrease in size and increase in isolation of the Dharwar outcrops as one passes south lead one to infer that the total area these beds now cover is not to be compared with that which they formerly occupied. All that now remain are relics of the troughs of a few of the compressed N.-S., or N.W.-S.E. folds.

Submergence.—During the next epoch, christened by Sir Thomas Holland the Purana, India sank again beneath the sea. Between this submergence and the end of the preceding one, the interval, known as the great Eparchaean interval, is thought .to have been so great as to exceed the time which has elapsed since the first records of life on the Earth up to the present day.

Purana Period.—Upon the submerged and highly inclined edges of the denuded Dharwar and gneissic rocks were deposited a great thickness of sand, clay and limestone, which in the Cuddapah area of Madras amounted to some 20,000 feet. The outcrop of these beds in this basin, including the portion concealed beneath some overlying younger deposits, occupies an area of about 14,000 square miles. The Purana sea covered the northern half of Madras, the Central Provinces and Bajputana, and stretched probably as far as the Himalaya. The Chail and Jaunsar series of the Simla area, for instance, are allotted by Dr. Pilgrim and Mr. West to this group ; the Jaunsar series in Jaunsar includes lava flows and volcanic ash. One may conjecture that much of the region of that part of the Himalaya lying to the south of the main range of snowy peaks was covered by this shallow sea, and that it was bounded to the north by a Tibetan continental area. Eastwards the Purana sea may have extended to Burma, in which province the system may be represented by some of the rocks of the broad belt which lies between the gneiss of the Ruby Mines area and the fossil-bearing strata of the Shan plateau. The beds containing the vast and rich deposits of iron ore in Orissa and its Feudatory States were at one time considered to belong to the Purana group, but are now assigned to a late phase of the Dharwar system.

Aravalli and Central Indian land: Vindhyan Period.—The next event seems to have been a corrugation of the sea floor by a movement having a direction N.W.-S.E. One of the effects of this movement was the initiation of the Aravalli range in Rajputana, and here the folding was comparatively intense ; elsewhere it appears to have been gentle and broad. This change ushered in the Vindhyan period and produced land in the Aravalli area ; this we deduce from the absence of the earliest Vindhyan sediments along the flanks of this old mountain chain. Over a broad belt stretching from the Malwa plateau and the so-called Vindhyan range to the Son valley the Lower Vindhyan deposits consist predominantly of sandstone and clay. In the Son valley they include volcanic ashes and more rarely lava flows, indicating the proximity of paroxysmal vents; similar volcanic deposits on a large scale are seen in the Rajputana desert west of Jodh- pur. Further away from the Aravalli land-area, over what is now part of Madras, the deposits comprise more limestone ; the lowest beds are, however, coarse conglomeratic sandstone and were evidently derived from a not very distant coast. There is evidence of such land in the form of a broad flat ridge, parallel to the Aravalli ridge and distant from it some 450 miles away to the south-east. This more southerly ridge extended from the region south of the Son valley through the Mandla, Seoni and Chhindwara districts of the Central Provinces, probably across Hyderabad to the Bombay Presidency. This ridge seems to have formed a barrier sufficient to separate two different basins of deposition, the Central Indian to the north and the Kurnool to the south. It is perhaps not entirely fortuitous that this ancient ridge, which has never since sunk beneath the ocean, still forms an important watershed in spite of the topographical changes produced by the Deccan lava ; from the Amarkantak section of this ridge rise the Narbada flowing ultimately westwards, the Son flowing northwards and north-eastwards, tributaries of the Mahanadi flowing to the south-east, and the Wainganga and Wardha flowing to the south.

Earliest Organic Remains.—The most interesting feature of the Lower Vindhyan deposits is the occurrence in them in south Indore of organic remains. These have been pronounced recently to be the chitinous shells of brachiopods allied to Cambrian forms of Acrothele ; on this account the beds have been assigned to the Cambrian. These, if rightly identified, are not only the earliest known fossils of the peninsula, but the only marine fossils found in the peninsular area at all, if we except a few isolated coastal deposits along the Coromandel and Malabar coasts, and a recently discovered exposure of Carboniferous in the Rewah State of Central India containing Productus and Spiriferina.

Extra-Peninsular Cambrian.—In extra-Peninsular parts of India the Cambrian is represented by definite marine faunas. In the Salt Range of the Punjab is a trilobite and brachiopod fauna of a very individualistic type but showing some affinity to Chinese, American and Australian forms. In the Spiti valley of the Punjab Himalaya is another and more extensive fauna which, like that of the Salt Range, contains no species definitely recognizable in any other part of the world ; it has, however, a marked resemblance to the Cambrian fauna of the Rocky Mountains. Its only link with the Salt Range is the trilobite, Redlichia noetlingi. In all probability these Cambrian beds extend through the Himalaya with possible interruptions at least as far as the frontier of Nepal. In places such beds, with any fossils they contained, may have been metamorphosed out of all recognition into schist and gneiss, but travellers passing through country in the line of strike of any known outcrop of Cambrian—or indeed of any of the other fossiliferous systems—would help to locate these elusive bands by keeping a look-out for organic remains or markings. Such relics are more especially noticeable in stream-courses where broken rock accumulates and where the action of water tends to make organic structure more conspicuous. More doubtful occurrences of Cambrian strata are those of Kashmir and the Hazara district of the Punjab,, but there is some reason for supposing that the Cambrian sea extended as far as Afghanistan and the Hindu Kush ; it did not reach as far as Europe, as the differences in the respective faunae prove. This Tethys Sea, as it has been called, had receded temporarily northwards from the Salt Range region, which became land—probably part of the peninsular area—during the latter part of the Cambrian period and remained so until the closing centuries of the Carboniferous.

Lower Palceozoic of the Peninsula.—In the northern half of the peninsula the Lower Vindhyan series is invariably succeeded by the Upper Vindhyan. This is a sandstone series and yields the pink or purplish sandstone so largely used for building purposes. It was especially so used by the Pathans and Moguls ; Akbar employed it in building his city of Fatehpur Sikri. The Upper Vindhyan must represent a later series of the Lower Palaeozoic, but has so far yielded no determinable organic remains ; its sandstones, however, exhibit records of ⁴⁴ fossil weather " in the form of ripple-marking, sun-cracks and rain-pitting. No representative of the Upper Vindhyan has so far been identified in the Himalaya.

Ordovician.—Ordovician sediments overlie the Cambrian in Spiti, and contain a brachiopod fauna showing a clear relationship to the fauna of the American Chazy or Trenton formations. Northwestwards the beds extend into Lahul and are probably found in Kashmir. To the south-east Ordovician beds have been recognized in British Garhwal, where they contain a Spiti fauna. The best development of the Ordovician is seen in the Northern Shan States of Burma ; here there is a rich fauna which, curiously enough, is much more closely related to that of North Europe than to those of the Himalaya (Spiti) and America. There seems to have been an effective barrier—presumably of land—between the Central Himalaya Ordo- vician sea and a sea stretching from North Burma through China and Siberia to Scandinavia. The central Himalaya sea was probably connected with the North American sea by way of Southern Europe through the forerunner of the present Mediterranean.

Silurian.—The Silurian follows the Ordovician in Spiti and Garhwal, and has been identified in Kashmir ; its fossils have elements in common with the American fauna, but their predominating resemblances are with north European forms. Silurian beds with a fauna including a rich assemblage of graptolites succeeds the Ordovician of Burma. Graptolites were minute polypes which grew in colonies arranged along a rachis or stalk. They exhibited a preference for muddy waters and for this reason their remains are more likely to be found on slate or shale than in sandstone or limestone. These remains form on rock surfaces black markings which remind one of the business part of a fret-saw arranged in various ways. This interesting and widely scattered group became extinct during this period. One continuous Silurian ocean seems to have spread round the northern hemisphere, including the old interior sea of North America, but to have been shut off from a precursor of the Pacific.

Devonian.—The only places where the Devonian system has been definitely identified are Chitral and Burma, but certain quartzites in Kashmir, Spiti and Garhwal, from their position above the Silurian, and some unfossiferous beds underlying the Trias of the Hazara district, may provisionally find a place here. The Chitral beds contain characteristic brachiopods and corals, while the Burma strata contain a rich assemblage of Devonian forms, including the characteristic and ubiquitous coral Calceola sandalina.

Carboniferous.—Carboniferous strata containing marine fossils have been found in the Salt Range, Kashmir, Spiti, Garhwal, Chitral and Rewah State ; the same beds also occur in the Northern Shan States, the Tenasserim division and probably in the intervening tracts in Burma. The discovery of Productus and Spiriferina in Rewah State, Central India, is interesting as pointing to the invasion by the early Carboniferous ocean of the peninsular or continental area to this extent ; this Carboniferous Tethys was still continuous with the European sea.

The Gondwana Continent.—Towards the end of the Carboniferous and the beginning of the Permian periods we find India forming part of a great southern continent stretching across the Arabian Sea and Indian Ocean, over the site of the Seychelles Islands to Madagascar and South Africa, and thence, south-westwards to South America and Antarctica ; to the south-east it was united to Australia and may have covered the rest of the Indian Ocean. To the north, girding the greater part of the Earth, was a latitudinal sea, the TetKys, of which the Mediterranean is a dwindled relic. The backbone of the Indian end of this old continent of Gondwanaland was the Aravalli Range, the oldest mountain range in India, which at that time must have formed a lofty snow-clad chain comparable to the modern Himalaya ; from its south-eastern flank flowed glaciers which fed streams in whose basins the coalfields of India subsequently accumulated. The cold, however, was not confined to the mountain heights, for evidence of the proximity of glaciers in the form of ice-scratched and facetted boulders and pebbles is widespread over the whole continent from Australia to the Argentine and the Antarctic regions, in the rocks belonging to this final phase of the Carboniferous, which in India is known as the Talchir. This extremely interesting series has a representative in the Himalaya in the Blaini stage of Simla. It was long suspected that the scratches found on a few of the boulders from the boulder- beds of this stage were the result of ice action. On two of the boulders, found in 1908, one by Sir Thomas Holland and the other by the writer of this article,1 the scratches and polished surfaces offer unmistakable evidence that the boulders are relics of ancient moraines and, according to Mr. R. D. Oldham's hypothesis, must have been carried out to sea upon floating ice and dropped upon the sea bottom. In no other way is it possible to explain satisfactorily the association of scattered boulders, some of them two or three feet across, with a fine silt whose bulk exceeds the total bulk of the boulders. A marine type of the Talchir boulder-bed is found in the Salt Range and there is evidence that these boulders, many of which are ice-scratched and facetted, came from the direction of Rajputana. The Kashmir area at this time was the scene of great volcanic activity and seems to have formed a volcanic archipelago whose islands poured out streams of lava—the Panjal Trap—and emitted quantities of ash which fell into the sea and became inter-stratified with marine lime-stones.

The Arctic climate of the boulder-beds was followed by one probably less severe but still cold. A dense undergrowth of ferns and cycads, apparently of Antarctic habit, covered the land and gave origin to the beds of coal which characterize the strata which succeed the boulder-beds; the best known of the ferns have been named Glossopteris and Gangamopteris. These coal-bearing stages are not found in the main mass of the Himalaya, their place being taken probably by slates with bands and lenticles of brown grit, forming what has been called the Infra-Krol, and a sandstone stage known as the Krol. In this Permian period, as the presence of the Gangamopteris flora shows, Kashmir was connected with the mainland to the south for a time, but soon sank and remained beneath the Tethys waters until early Tertiary times. To the east of Kashmir the Himalaya appears to have formed part of the continent for a period of sufficient length to allow of the denudation of several thousand feet of beds ; during the following Triassic period, however, it was again submerged.

The Tethys Sea.—Meanwhile the Tethys Sea persisted as the northern boundary of Gondwanaland and, although much reduced by the upheaval into land of a large part of China, in Triassic times stretched from north-east of Dar jeeling through Kumaon and southern Tibet, Garhwal and Spiti into Kashmir and westwards into Europe across the Pamirs, Bokhara, Afghanistan and Baluchistan. Its course can be traced by deposits with Triassic fossils. North of it was a land- mass covering the greater part of China, Siberia and north Bussia, to which the name of Angaraland has been given. Either a southerly prolongation of the Tethys or a separate sea occupied the greater part of Burma, for we find Triassic coastal deposits along the Arakan Yoma ; to the west of the Yoma was land which in all probability was continuous with the Madras area over what is now the Bay of Bengal.

Separation of Africa from Asia.—It was probably during the Permian period that an arm of the Tethys commenced to extend slowly southwards along the Red Sea over that portion of the Gond- wana continent which now forms the eastern margin of Africa. Whatever its time of commencement may have been, we know that during the following Triassic period it had penetrated as far as the north of Madagascar.

Jurassic.—During the next period, the Jurassic, the face of Asia changed considerably and the old Gondwana continent began to break up. The sea-arm from the Tethys pursued its way to form the Mozambique channel, separating Madagascar completely (for a time at least) from Africa, and then seems to have expanded eastwards to produce the southern and eastern parts of the Indian Ocean including the Bay of Bengal. The land connection between India and Madagascar was maintained still across the site of the present Arabian Sea. In some deposits along the east coast of Madras occur a few coastal fossils of Jurassic age, including an ammonite found in Madagascar and South Africa. This occurrence affords us a dim picture—amply confirmed by evidence from later deposits—of a free sea connection along a continuous coast from this part of India to Madagascar. The Coromandel coast and a large portion of the Indian Ocean including the Bay of Bengal, therefore, date from the Jurassic era. Meanwhile southern Tibet was occupied by the dwindled Tethys which still maintained its connection with the European sea. The Salt Range once more, as also parts of Baluchistan, southern Afghanistan and Persia, lay beneath this sea. In Burma sea and land seem to have changed places during this period, the Arakan Yoma forming a coast to the newly formed Bay of Bengal, while the eastern parts of Burma became land. This change was brought about bv an E.-W. earth movement, initiating the N.-S. topography of Burma which has persisted to this day.

Cretaceous.—Of Cretaceous deposits we find patches along the east coast of Madras just as we do of Jurassic. From these younger beds, however, an extensive fauna has been obtained and this is identical not only with that of similar beds in Madagascar and South Africa but also with that of beds of the same age in the Shillong plateau of Assam, showing that the old Gondwana coast-line not only persisted from Madras to Madagascar but extended north-eastwards during the Cretaceous period as far as the Shillong plateau. In the north the Cretaceous period was characterized by an extension of the sea over a large part of Tibet, but the southern shore still remained close to the present Indo-Tibetan frontier, as the coastal nature of the south Tibetan Cretaceous deposits shows. The presence of this sea can be identified along the northern frontier of Sikkim.

Deccan Trap.—Towards the end of Cretaceous times there commenced a period of disturbance and earth movement which brought about further striking changes in the geography of Gondwanaland. A movement from the north caused the recession of the already dwindled Tethys from Central Asia, and initiated the Himalayan chain and the Tibetan plateau along the northern coast of the continent. At the same time a continuation of the movement from the east elevated the Shan plateau and raised the Arakan Yoma above the level of the sea. These movements in their early stages were accompanied by the outpouring of floods of basaltic lava on a colossal scale. This lava, the Deccan Trap, was ejected through fissures in the Earth's crust and issued in such quantities that the relics to-day, after millions of years of denudation, cover over 200,000 square miles in the Bombay Presidency, Hyderabad, Berar and the Central Provinces and Central India. Since the eruptions took place along the flank of the Aravalli watershed, they did not seriously disturb the drainage scheme of the area.

Early Tertiary.—During early Tertiary times the slow rise of the Himalaya along the Tethys coast produced along its southern flank a gulf which extended as far south-east as the meridian of Lansdowne. At its north-western end it curved southwards to enter the Arabian Sea which was formed about this time in the following way. As a result probably of earth movement, a large block of Gondwanaland west of what is now the Bombay coast was broken off and submerged beneath the waves. The age of the Malabar coast and the Arabian Sea is, therefore, probably early Tertiary ; some small coastal deposits near Quilon with early Tertiary fossils confirm this. The straightness of this coast-line and its lack of indentation are due to this fracture or faulting and to its comparatively recent age. The same cause brought about the truncation of the Aravalli drainage, and for this reason all the important rivers of Madras and Southern Bombay are easterly flowing and rise within a few miles of the west coast; they are in factlbut the lower portions of older rivers which rose in the old Aravalli watershed further west. The disjunction of the submerged part of the continent was assisted by faults in other directions ; one of these seems to have coincided with the southern boundary of the Kathiawar Peninsula and to have initiated the Narbada, the middle section of which has an unusually straight course. The Eocene or early Tertiary epoch along the Indo-Tibetan border was one of great volcanic activity, beginning with an intrusion of granite into the rocks of southern Tibet. Afterwards there followed the outpouring of basic lava in Ladakh, Ngari Khorsum and western Tibet. This volcanic phase seems to have culminated in the region around Lake Manasarowar.

Petroleum Deposits.—In the early Tertiary gulf of the Punjab were accumulated the petroleum deposits which are now being exploited in the Pindigheb district. Similar gulfs, in which petroleum deposits were formed a little later on, were also produced further east, one passing up into Assam from the Bay of Bengal, and the other up what are now the Irrawaddy and Chindwin valleys. Thus originated the Digboi oilfield of Upper Assam and the rich oilfields of Yenangyaung and Singu in Burma.

Initiation of Rivers.—As the Himalayan chain continued to rise, the gulf along its base became silted up and gave place to a river, the lower section of which coincided with part of the modern Indus. The gulf in Burma also silted up and was also replaced by a river coinciding with the present Irrawaddy and its chief tributary, the Chindwin. This old Chindwin-Irrawaddy river is thought to have been continuous with the Tibetan part of the Brahmaputra, the Tsangpo, which was subsequently captured by the backward cutting of the Brahmaputra proper, i.e., the Assam Brahmaputra. The compressional movement which piled up the Himalayan chain produced a deep trough in front of it which was simultaneously filled and is still being filled with river sediments. The movement is, in all probability, still persisting and the Himalaya still rising.

Himalayan Gorges.—The great depth and steepness of most of the Himalayan gorges are witnesses to the recent nature of the uplift. The position of the longitudinal gorges may be looked upon as having been determined in nearly all cases solely by the presence of a more easily eroded band or rock. Like the transverse gorges they are the result of denudation. They do not normally occupy the troughs of the folds ; in fact they are more likely to have been initiated along the arches or crests which would split open to the weather more than the troughs as a result of the folding movement. The transverse drainage was initiated when the Himalayan region first rose from beneath the Tethys Sea. The deepening of these transverse valleys kept pace with the uplift of the range and its plication. As the uplift increased so did the speed of the transverse rivers and consequently their power of erosion. The effect of the upheaval was merely to make them bite the deeper into the rocks along their beds. That is why the courses of so many of the Himalayan rivers run right across some of the loftiest ranges, one after the other, the reason being that the rivers were there long before the ranges reached their present altitudes. To affect the course of a river, uplift or depression would have to be comparatively rapid. An earthquake, a landslip or the ejection of volcanic material would be likely to produce far greater changes than the slow diastrophic uplift of even such a mighty mass as the Himalaya, which, though it has attained an average altitude of something like 19,000 feet above the Granges plain, has taken something of the order of six: or seven million years to do it in.

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