Provincial Geographies of India/Volume 4/Chapter 7

by Herbert Thirkell White

CHAPTER VII

GEOLOGY

A study of the present-day topography of a country would be incomplete without an enquiry into the steps by which that topography had been produced. The contemplation of the coast-line, mountain ranges, valleys, river-systems, fauna, flora and climate of any particular region awakes in us a keen curiosity as to the various changes in configuration and meteorological conditions which have made up the past history of that region. No contemporaneous brain has penned such a history, whose torn and disfigured pages are the rocks from which we try to reconstruct former conditions.

The first chapter of this most ancient book of Nature, in the case of Burma, consists of a belt of gneiss, a quartz-bearing rock which, under the influence of enormous pressure and high temperature, has assumed a banded streaky texture. This belt, commencing near Mandalay, extends northwards and north-westwards through the ruby mines of Mogôk into the country north of Bhamo and into the Chinese province of Yünnan; above Mogôk ranges formed of this gneiss culminate in the Taungme peak (7544 feet above sea-level) . Folded up with the gneiss are beds of limestone in which the celebrated Burmese rubies and sapphires are found, but these pages of Burma's geological history are so blotted and blurred that we cannot decipher the conditions under which the limestones and some of the gneiss were produced. The former may represent sediments laid down on an ancient sea-bed, or they may be deposits formed after the consolidation of the surrounding rocks by percolating solutions of carbonate of lime. Some of the gneiss is solidified molten material, but

Fig. 23. Geological map of the Northern Shan States.

whether it solidified underground as bosses and dykes or was ejected at the surface as volcanic matter it has not yet been possible to say. Much of this gneiss has been referred to the lowest horizons of the Archaean, the oldest geological system known, whilst other different types, together with the ruby-bearing crystalline limestones, show distinct resemblances to the Dharwar rocks of the Indian peninsula, which are metamorphosed sedimentary deposits of later Archæan age. A similar gneiss was observed in the Yamèthin district, and it is surmised that a more or less continuous belt of this rock extends from Yünnan to the vicinity of Moulmein. Intersecting in every direction the ancient Archæan gneisses are veins of granite which in a molten state has been forced into fissures.

The next oldest rocks have so far yielded no fossils, and must belong either to the Cambrian—i.e. the oldest known fossil-bearing system—or to an intermediate system between the Cambrian and the more ancient Archaean, or to both. They comprise what has been called the Tawngpeng system in the Shan States and the Mergui beds of Tenasserim. Whatever their precise age may be, their most characteristic feature is the complete absence of lime, the rocks consisting of schists, various altered forms of sandstone and clay, and an interesting series of lava flows and volcanic ashes ejected from neighbouring vents. The lavas in the Shan States strongly resemble similar lavas found in the Malani district of the Jodhpur State in Rajputana and are of much the same age. Amidst the volcanic ashes and lavas occur the important lead, silver, zinc and copper deposits of Bawdwin. In Tenasserim the Mergui beds, which probably form a more or less continuous belt with those of the Shan States, are also associated with rich mineral deposits, but in this case of wolfram and tin^[1]. The Mergui beds are no greater in bulk than the granite which has been intruded into them. This granite has been traced as far north as Yamèthin and is very probably continuous with a broad belt of the same rock in the Kyauksè district. A different kind of granite containing the mineral tourmaline has been injected into the schists of the Tawngpeng system in the Ruby Mines sub-division and is also found associated with the Mergui beds of Tenasserim.

The Tawngpeng rocks formed a continuous floor on which the fossiliferous sediments of the Palaeozoic ocean were deposited. There is some interesting evidence by which it seems probable that the Tawngpeng rocks in the Shan States area projected here and there above the surface of this ocean in the form of islands, for some of the sediments of the Palaeozoic ocean thin out in a remarkable way as they approach these islands, and frequently overlap older sediments below. An example of one of these islands is the mountain mass of Loi Leng.

The records which follow thrill one with their clearness and their absorbing interest. They are scored on rocks which contain a rich fauna generally similar to that found in the rocks of North Wales and on that account termed the Ordovician system. The largest exposure of these rocks is to be seen about 11 or 12 miles east of Mandalay. Their fauna includes specimens of the stalked, bud-like, extinct animals known as cystids, many species of brachiopods—bivalve shell-fish of which one shell covers the back and the other the belly—including more than one identical with European forms, and many remains of the curious, extinct, primitive crustaceans known as trilobites. The earlier sediments of this system are homogeneous over wide areas in the Shan States, and indicate an open sea of uniform but no great depth, with a coast-line somewhere to the west. The later deposits are crowded with shell fragments and the detached eyes of trilobites, and were evidently laid down in a sea teeming with life. Perhaps the most interesting point about the Burmese Ordovician, however, is the relationship of its fauna to that of other

Fig. 25. Ordovician Trilobites from the Shan States; the third specimen has been distorted by pressure.

Fig. 25. Ordovician brachiopods from the Shan States.

Ordovician exposures. It is found that this fauna resembles much more closely the Ordovician fauna of North Europe, especially of the Baltic provinces, than it does that of the central Himalaya. The Ordovician of the Himalaya in Spiti and Kumaon contains no cystids and extremely few trilobites, and its fauna generally resembles that of the American beds of the same age. We are constrained, therefore, to make the interesting deduction that in Ordovician times the Arctic sea which covered what is now Northern Europe, extended into Burma as far south as latitude 21° N., and was separated from the sea which covered the central Himalaya. The former sea spread over central and north-eastern China, but of its southern limit we know nothing as yet.

Lying upon the Ordovician are shales, sandstones and limestones of no less interest and importance. These beds, from the character of their rich fauna, belong to a system known as the Silurian, after an area in South Wales in which they were first studied. As in the Ordovician, trilobites and brachiopods are common, but the occurrence of most interest is that of the peculiar rod-like colonies of animals known as graptolites; these were found for the first time in Asia by Mr T. D. La Touche in December, 1899. The discrepancy between the Burmese fauna and that of the central Himalaya and America, noted in the case of the preceding system, is still more strongly marked in the Silurian. About 87 per cent, of the Burmese forms are identical with or allied to European forms, while only 35 per cent, are so related to America; only seven forms are common to Burma and the central Himalaya, and these are all ubiquitous forms. The Himalayan forms are allied to American and European in practically equal proportions. The central Himalayan fauna includes no graptolites and is predominated by corals, most of which have marked American affinities, while the Burmese fauna is characterized by abundant graptolites, several of which are identical with those of Great Britain, and only one coral. We are justified in concluding, therefore, that the two Ordovician oceans remained separate from each other throughout Silurian times also. The southern extension of

Fig. 26. Fault Scarp at Pong Wo, Northern Shan States.

the Silurian ocean which covered the Shan States is not yet known.

The uppermost Silurian beds pass up into a great limestone system which has been termed the Plateau Limestone from the fact that it forms the great undulating plateau of the Shan States. Visitors to Maymyo will be familiar with the typical scenery belonging to this system, rolling uplands covered by a thick mantle of poor red clay soil on which the common bracken fern nourishes, shallow valleys or precipitous gorges, and steep cliff-scarps due to faulting. The limestone, when freshly broken, has, like so many limestones, a fœtid smell. It usually includes a proportion of carbonate of magnesia, and in fact varies from a pure lime carbonate to a true dolomite. The rock is characterized by a network of calcite veins, which have evidently filled an innumerable series of fissures, and give the rock such an unstable texture that one blow from a hammer will often shatter a small boulder into fragments, a quality much appreciated by those in quest of road-metal or railway ballast; as the limestone has not been excessively folded this peculiar fracture structure must be regarded as the direct result of pressure, exerted after the consolidation of the limestone and caused by the great earth movements which commenced just before the Tertiary period. Fossils are not frequent but from the small assemblage of forms so far collected, the great bulk of the Plateau Limestone belongs to the Devonian system, and its fauna, incomplete as it is, seems to have more in common with the Devonian of Europe than with that of America. The Plateau Limestone in all probability extends continuously to Moulmein and Tenasserim, and similar limestones have been described in Yunnan and other parts of China. Mr La Touche concluded that the Plateau Limestone was formed under conditions similar to those of modern coral reefs, and that some of the ranges, such as Loi Leng and the Loi Pan-Loi Twang range, may have been islands in the Devonian sea.

Here and there upon the plateau we find small patches of a limestone which differs in many ways from the great bulk of the beds below, and in some places contains abundant fossils. An examination of the latter shows that these patches are younger than the underlying massive limestone, and correspond to the middle and upper Permo-Carboniferous of the Salt Range in the Punjab and of the Ural Mountains. The most significant points are the identity of many of the species with central Himalayan fossils, and of a few with Malayan forms, and a closer resemblance of the fauna to that of America. The Permo-Carboniferous ocean of the Shan States seems to have been connected in one direction with that of Malaysia, and in another direction with that of the central Himalaya and America.

During the next phase the coral reefs of the Devonian and Permo-Carboniferous, at least in the Shan States, were raised by a gentle earth-movement, which caused the sea to retreat leaving a land surface subject to the denuding agencies of the atmosphere. This land phase in the Shan States lasted throughout all but the lower part of the Permian and all but the uppermost part of the Triassic periods. Towards the end of this phase we find evidence of sea along the west of Burma, occupying what are now the Arakan Yoma and the Naga Hills, and to the north in Yunnan where the presence of beds of salt and coal points to shallow water conditions. After the coral-reef land of the Shan States had been worn by rain, rivers and other natural agencies into ridges and hollows, a gentle depression of the whole caused an invasion of the sea, which filled up the hollows with fine sediments. Some of these hollows maintained a connection with the open sea while others seem to have been more or less completely cut off as salt lakes. The amenities of aquatic life, therefore, varied considerably, and we find in some localities badly developed, stunted forms typical of restricted conditions, and in more salubrious spots well-favoured massive forms. These fossils have been identified as belonging to the uppermost Trias (Rhaetic stage), and a similar fauna has been found to the north in Yünnan and to the south in the Malay States and Sumatra.

The Shan States remained part of a shallow sea extending into China throughout the next period, the Jurassic, but was raised to dry land for the last time at its close. The Jurassic sea probably extended westward, covering the area now occupied by the Arakan Yoma and Naga Hills, but seems to have had no connection with a contemporaneous sea spreading at that time over the central Himalaya and Cutch.

The period which follows the Jurassic is known as the Cretaceous and precedes the Tertiary. During the Cretaceous period a widespread system of earth-movements was initiated which lasted throughout the Tertiary and does not appear to have quite ceased at the present day. The configuration of Asia was greatly affected thereby and the present system of mountains and valleys in Burma is entirely due to one of these corrugating movements. India, before these movements took place, formed part of a vast continent known as Gondwanaland which included a large part of Africa. One portion of the coast-line of this continent coincided approximately with the present east coast of India, but extended north-eastwards along the southern margin of the Shillong plateau and Mikir Hills up to the north-east corner of Assam, where it probably curved round and joined the western margin of the Shan plateau, passing southwards in a sigmoidal curve close to Moulmein and the present coast of Tenasserim and Malaysia. The sea bordering this coast was in fact an enlarged precursor of the present Bay of Bengal. At the same time another sea covered Tibet and stretched as far eastwards as Sikkim. Whether these two seas were ever connected is doubtful. The only part of Burma which formed dry land at this time, therefore, comprised Tenasserim, Karenni, the southern and northern Shan States, and the Kachin country, connected through Yunnan with the Gondwana continent of India and Africa. The Chindwin valley, the whole of the Irrawaddy valley excepting the uppermost portion, and probably the whole of the Naga Hills and Arakan Yoma, as well as the Andaman and Nicobar Islands, were covered by the sea which formed the ancient Bay of Bengal.

Two of the earth-movements mentioned above have now to be considered, one from the north and one from the east, both advancing towards the centre of the Indian peninsula. By the movement from the north the northern margin of the Gondwana continent was folded again and again into the mighty mass of the Himalaya, the Tibetan sea retreating westwards until central Asia became dry land. In front and along the foot of this mountain mass was a deep trough, which at first formed a long gulf, but which subsequently became filled up with river sediments, and now coincides with the Ganges valley and portions of the valleys of the Indus tributaries. There is some evidence to show that behind the mountain mass was another less pronounced trough separating the mountain range from the Tibetan table-land, and that this was also occupied successively by a gulf and a river. The movement from the east produced an almost exactly similar effect upon Burma. The Shan plateau may be looked upon as corresponding to the lofty Tibetan table-land, the Arakan Yoma and Naga Hills take the place of the Himalaya, the Chindwin-Irrawaddy valley is the smaller trough behind the mountain mass, and the Arakan coast and the valley of the Meghna river which originally stretched into Upper Assam represent the large trough in front. Each of the two troughs, as in the case of the Himalayan troughs, was occupied successively by a gulf and a river, the rivers being the Chindwin-Irrawaddy (for the Chindwin was originally the head-waters of the main river) and the old Meghna. The point of interference between the two great movements just described was the north-eastern corner of Assam; here the mountain ranges of the two systems veer round till they confront one another in more or less parallel lines. One small difference between the two areas is that, whereas

Fig. 27. The Chindwin River.

the Himalaya is a simple curve convex towards the direction of the compressional force, the Naga Hills and Arakan Yoma with the Andamans and Nicobars form a well-marked double S-shaped curve.

It is difficult to say precisely when the Naga Hills and Arakan Yoma first began to appear from beneath the waters of the old Bay of Bengal, but the Meghna and ChindwinIrrawaddy gulfs seem to have been separated off from each other early in the Tertiary period; in these two gulfs were deposited the oil-bearing sediments which to-day are being tapped at various spots along the Arakan coast and at different localities in the Irrawaddy and Chindwin valleys. The movement from the east corrugated the floors of the gulfs and caused their silting up. A river in each case succeeded them and its sediments also were folded by the continued movement. This east-to-west movement is the key to all the present topography of Burma, and is the reason for the north-to-south sigmoidal direction of its mountain ranges and valleys. The Andaman and Nicobar Islands are an extension of the Arakan Yoma and are connected with it by a submarine shelf or ridge. The Chindwin river is thought to have been continuous at the end of the Tertiary period with the Tsanpo or Upper Brahmaputra, which was subsequently captured by the Assam Brahmaputra.

At the close of the "gulfs" period several volcanoes made their appearance, chief among which was the mysterious and legend-haunted hill of Mt Popa, which rises in lofty solitude from the plains of Myingyan. Its earliest lava flows were covered by river sediments, but subsequent ejections of lavas and ashes have piled a cone nearly 5000 feet above sea-level. Flows of a different type occurred from other vents in the immediate neighbourhood, and several craters, thought to have been produced by gigantic explosions, are to be seen along the banks of the Lower Chindwin.

Fig. 28. Mt Popa.

Fig. 29. "Mud volcano," Minbu.

The so-called "mud volcanoes" of Cheduba Island, Minbu and other places are of a totally different category and origin. They are not volcanoes in the correct sense of the term, but mere ejections of salt mud thrown out by the force of hydrocarbon gases and frequently accompanied by petroleum. They are, in fact, nothing more than the accompaniments of gas seepages, and are intimately associated with occurrences of oil. Through the stopping up of the vents, these mud eruptions are sometimes of explosive violence, and the friction of colliding stones with one another may set fire to the inflammable gas, producing a result still more closely resembling the fiery eruption of a true volcano. This has happened more than once in the case of the large "mud volcano" in the island of Cheduba off the Arakan coast.

The Tertiary plateau, as well as the higher-lying Shan plateau, is covered with a mantle of red clayey silt which passes here and there into a deposit of gravel. The still more recent clays and sands, which have produced and are still producing the deltas of the Irrawaddy and Sittang, constitute part of the last-written and still incomplete chapter in the history of Burma. What the end of this chapter will be and what other chapters will follow, who would dare to say?

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1. See p. 74.