372 EEPORT— 1882. 



ascertain that the rocks cropping out through the sand do not differ 

 geologically from those which compose the border mountains of the desert, 

 we should be induced to conclude that the one is merely a continuation of 

 the other. Therefore let us throw a glance on the mountains which form 

 the boundaries of the desert, beginning with the northern or Siberian side. 



Here, I have been able myself to ascertain the PalEeozoic age of the 

 Altai and the Sayan ranges,^ consisting chiefly of clay-slate, limestone, por- 

 phyry, &c., and it is probable that the Jablonovo'i chain, which is an 

 eastern continuation of the Sayan, belongs equally to the same age. 



The latest explorations of the Thian-Shan, or celestial range, the 

 various ramifications of which form the southern and western boundary 

 of the Gobi, tend also to prove that they are referable to an old geological 

 formation. Colonel Prejevalsky, who more than once crossed the moun- 

 tains of the south-eastern part of this boundary, mentions as the chief 

 rocks composing them, granite, syenite, granulite, porphyry, diorite, 

 mica-schist, clay-slate (ThonscMefer), chloritic schist and coal deposits. 

 These are the geological elements which, according to Colonel Preje- 

 valsky, constitute many of the marginal mountain ranges which he visited 

 between Kalgau and the lake Kuku-Nor. He mentions very extensive 

 coal-deposits in the mountains of Alashan, rising beyond 10,000 feet, as 

 well as in the mountains, which in Northern Tibet form the eastern 

 boundary of the Kuku-Nor. Those facts prove that the mountain range, 

 representing the south-eastern boundary of the Gobi (from Kalgan to the 

 Kuku-Nor) are composed of rocks which very likely belong to ancient 

 geological formations. 



A similar conclusion may be equally applied, with great probability, to 

 the long chain of Chingan, which can be considered as the eastern boun- 

 dary of the Gobi, and separates Mongolia from Manchuria ; for this chain 

 is intimately connected with the mountains of Inshan, one of the south- 

 eastern marginal mountains of the Gobi, which Colonel Prejevalsky 

 found composed of granite. At all events, all those mountains may be 

 considered as the eastern extremity of the long chain of Kuenlun, which, 

 according to Baron von Richthofen, is the largest and altogether the 

 oldest mountain-chain of whole Asia. 



We have, consequently, no lack of arguments in favour of a very 

 ancient formation of the Gobi, and we can admit that at the time when 

 -the mountains which surround the desert were upraised, the immense 

 space included in the interior precinct remained much lower, but still 

 sufficiently high to form one of the loftiest table-lands of the world, 

 the average height of which. Colonel Prejevalsky estimates to be near 

 4,000 feet (1285 metres), with local depressions sinking to about 3,000 

 feet. 



It is, therefore, probable that after its upheaval, this large surface has 

 never been covered by the sea, as little as the Sahara- Lybian desert 

 since the cretaceous and tertiary periods, or the Turkestan deserts since 

 the Palaeozoic epoch. Once more, in the Gobi, as in the other two deserts, 

 the sand-accumulations had nothing to do with marine deposits ; they 

 were chiefly produced by atmospheric agencies, and as far as the Gobi is 

 concerned, the frequent siliceous rocks, as granite, syenite, gneiss, &c., 

 were particularly apt to yield sufficient materials to the formation of 

 quartz sands. 



• Tchihatchef, Voyage scientifique dans VAltai oriental et les parties adjacentes 

 de la frontiere de Chine. Paris : 1845. 



