of the Annamite Chain, where marine conditions appear to have been maintained up to the end of the Permian, most of Indo-China was occupied by lagoons and islands, often the scenes of widespread eruptions of porphyrites, dolerites and andesites. Then followed the more extensive Upper Uralian-Permian transgression, abruptly at first and more progressive afterwards, covering the greater part of Indo-China but leaving unsubmerged various relics of the old massifs; to the north and west, large parts of Yunnan, the Shan States and Siam were flooded, while further south its fossiliferous deposits have been found in the Malay States and the whole of the central border of the Archipelago almost as far east as Serang. In the geosynclinal areas immense thicknesses of limestones occur but rocks of a neritic character are common elsewhere, while continental or lagunar formations are remarkable for their abundance of felspathic sandstones, sandstones, arkoses, conglomerates, shales, marls and carbonaceous shales which often contain intercalations of volcanic rocks usually of a rhyolitic character. These rhyolites may in some cases extend into the Trias, as, for example, in Tongking, where Prof. E. Patte found rhyolitic tuffs containing Myophoria. Outside certain parts of Eastern Tongking, the southern districts of the Upper Laos and the Lower Laos, where they are typically marine, the Permian strata of IndoChina, partake rather of these characters, though marine intercalations with brachiopods occasionally occur in the sandy-argillaceous sequences.1 It is not possible in the existing state of knowledge to synchronise exactly the more intimate changes of land and sea in the southern Chinese basin proper with those of Indo-China, and still less with those of the Shan States and Tenasserim, where large areas are still unmapped and further comparative palæontological studies have still to be undertaken. But in the case of Yunnan and Indo-China there are broad features common to both which can only be interpreted as the results of the operation of identical causes. Apart from the close relationships of the marine faunas, with the possible exception of those of the Lower Yangsinian, there is correspondence in the ages of the volcanic rocks, between the lithological characters of the deposits themselves, between the physical conditions under 1 Bull. Serv. géol. Indo-Chine, Vol. XIX, Fasc. 2, pp. 22-34, (1931). K which they were laid down, and, particularly, in the common occurrence of the Gigantopteris nicotinafolia flora in both regions. We have already seen that various parts of South China remained as islands above the surface of the Yangsinian sea and that such conditions became accentuated further to the west in Lopingian times. In the case of Indo-China we find independent evidence of a similar character, close enough to be impressive. M. Fromaget has proved that rocks of the neritic and continental types with their related lava flows, and in extreme cases the latter alone, may take the place of the Uralo-Permian limestones. He believes, moreover, that they were deposited in shallow lagoons, or around the shores of islands of varying sizes which existed outside the main geosynclinal channels, from the Middle Uralian to the end of Permian times, both as stages for the successive displays of volcanic activity which characterise the whole region and as the grounds on which flourished a flora whose affinities are with northern regions only.1 Gigantopteris nicotinafolia occurs in coal seams associated with sandstones, shales and tuffs of Lower or Middle Permian age in the Nam On valley, a tributary of the Mekong, to the north of Luang Prabang in the Upper Laos.2 This is the most westerly occurrence of the flora yet discovered and it is within 150 miles of the Southern Shan frontier. (See Pl. 29). It may be recalled here that the Lopingian limestones of the Southern Shan States betray their shallow water formation. They are clastic rocks, interbedded with shales containing varying quantities of calcareous matter, the products of the erosion of older rocks rather than true limestones of zoogenetic origin. From at least Middle Permian times onwards (using the term in the sense adopted by Grabau), there is therefore much support for de Launay's ideas, adopted and quoted by M. M. Blondel and Fromaget, that the Shan-Yunnan-Indo-Chinese region then formed an archipelago of islands, girdled by coral reefs and often possessing active volcanoes, somewhat on the lines of existing conditions in the Malay Archipelago today. These islands laid off the southern and western coasts of Cathaysia, which at any rate for part of the period concerned, formed a portion of the "Continent Eurasiatique " of the French writers.3 1 Bull. Soc. géol. France, 5th Serie, Tome 4, pp. 125-126, 156-157, (1934). 2 Bull. Soc. géol. France, 5th Serie, Tome 4, pp. 140-142, (1934). 3 Bull. Serv. géol. Indo-Chine, Vol. XIX, Fasc. 2, p. 42, (1931). I have considered it necessary to outline these palæogeographical features from Yangsinian times onwards because in the terrestrial volcanic rocks of Yunnan and in the contiThe Middle Permian and the Final Tendency nental facies of the Lopingian of Eastern to Uplift. Yunnan and Western Kueichow, there is to be found the beginning of the final tendency to uplift and emergence which, accentuated in Red Bed and later times, resulted in the final disappearance of the sea and the freeing of the whole of South-Western China from marine deposition towards the close of the Trias, with the exception of a brief and limited incursion of the Shan Jurassic gulf into the extreme west of Yunnan. The term “final tendency to uplift", is chosen advisedly, for similar events had occurred in earlier geological epochs across the same terrain, in the Lower Devonian and Middle Carboniferous, for example, though not on so grand a scale nor continued to so definite a conclusion. Events associated the Red Beds. The formation of the salt-bearing Red Beds of Yunnan and Szechuan marked another stage in the orderly procession of these events. The basaltic regions of Central with the Formation of Yunnan and adjoining parts of Szechuan, another great island in the Lopingian sea, were subjected to severe and prolonged erosion, the products of which doubtless helped to build the clastic sediments of the near-by Gigantopteris coal measures and of the shallow straits further east still. There, above the limestones with Lyttonia and Oldhamia, are red shales, sandstones and further thin limestones, part of which are now known to belong to the uppermost Permian, but the bulk of which are of Lower Triassic age. In some parts of Central Yunnan long continued denudation removed the whole thickness of the volcanic rocks as well as the underlying Permian and Carboniferous strata, so that the basal conglomerates of the Red Beds may rest on any horizon from that of the igneous rocks downwards. The lower, saliferous Red Beds which follow them are the results of deposition and precipitation in an enclosed basin, the dessicated remains of an inland sea which became isolated from its oceanic connections to the south-east. Whether the sea in question was originally part of the Yehlangian or Upper Permian transgression as visualized by Grabau and Ting, or, whether it is more correctly regarded, as Hirth has postulated, as a connection of an early Triassic inundation, is a question which, in my opinion, is still an open one. Be this as it may, later events in Central Yunnan belong to lacustrine Norian-Rhætic and continental regimes culminating in the coal measures. In Szechuan the Trias is followed by coal-bearing beds of Rhætic-Lower Jurassic age, but there the sinking of the interior of the basin led to the birth of the great Wealden lakes and the Lower Cretaceous fresh water Red Beds. The possibility of part of these being of Jurassic age must not, however, be overlooked. Should the second of the two views eventually prove correct then the unconformity at the base of the Red Beds will represent a great physical hiatus between the Paleozoic and Mesozoic eras, while in late Triassic time, the area formed a land to the east and west of which laid the two marine gulfs of Amichou-Kueichow and Yunnan-i, respectively, to the description of which it is hoped to return in a later paper of this series. Saurin studied the petrological characters of the Red Beds themselves and concluded that their elements were derived from the slow erosion in place of a granitic and metamorphic region, and such strata are occasionally, though by no means always, seen to form the floor below them.1 Conclusion. Red Beds of Triassic age have been found to occur in Szechuan, Indo-China and Siam. With the Red Beds of the Shan States those of Yunnan have nothing in common, beyond certain superficial lithological resemblances, with the proviso that it is safe to assume a Jurassic age for the whole of the Namyau Series on the strength of the fossils from the limestone bands, an assumption which I find it difficult to accept. La Touche pointed out long ago that the continental conditions prevalent in China did not reach the Shan States until the Jurassic period, but they were not, whatever age range they may possess, spread over the "whole eastern border of Gondwanaland", as he thought. It is now apparent that the whole sequence of lagunary and lacustrine, continental and sub-continental deposits, generally of red bed types, which may be of any age from the Moscovian to the Lower Cretaceous and occur from the coast of Southern Burma eastwards through Siam and the Shan States, Indo-China, Yunnan, Szechuan and beyond, can no longer be regarded as coming within the territory of Gondwanaland or be classified in any way as "Gondwanas". They are to be associated rather with the northern 1 Bull. Soc. géol. France, 5th Sér., Tome 3, p. 456, (1933). continental element, or with one or other of the sub-continents into which it had divided before early Mesozoic times. The future detailed study of these rocks, particularly in the Shan States and Yunnan, cannot fail to yield results of the greatest importance in the elucidation of the many unsolved problems of this border-land "where two impinging continental margins have extended themselves alternately over an intervening and fluctuating sea during the interplay of Laurasia and Gondwana in the Mesozoic era ".1 V. ACKNOWLEDGMENTS AND NOTE ON THE MAPS. Every student of Chinese geology is under an obligation to the geologists of the Geological Survey of China and to those others whose writings, with theirs, illuminate the publications of the Geological Society of China. The preparation of this contribution to the geology of Yunnan would have been impossible without the use of the works of the authors listed in the bibliography which follows, and especially to those of the late Drs. V. K. Ting and Y. T. Chao and of A. W. Grabau, T. K. Huang, C. Y. Lee and Y. L. Wang. In addition to this. I have received courteous assistance in connection with particular problems, from the following authorities during the preparation of this paper, and to each of them my thanks are here formally expressed-Prof. G. B. Barbour, Cincinnati; Prof. Wilhelm Credner, Munich; Prof. T. G. Halle, Stockholm; Prof. Arnold Heim, Zurich; Dr. Karl Krejci-Graf, Berlin; Prof. Etienne Patte, Poitiers; Dr. F. R. Cowper Reed, Cambridge and Dr. Eberhard Wirth, Celle in Hanover. The geological sketch map (Pl. 30) is based entirely on traverses and can only be taken to illustrate the broad outlines of the chief Notes on the Maps. exception of: formations in a very diagrammatic manner. It is a result of my own field work with the (a) the portion north and north-west of a line through Yunlung Chou, Tengch'uan Chou and Chingchiangkai which is added from the observations of Gregory and Credner. As they do not agree on the ages of the strata exposed between Lanping and Chiench'uan Chou, this section is particularly doubtful. A. L. Du Toit, p. 296. |