CHINA, MONGOLIA, AND JAPAN. 75 



hundred feet high, and remarkable for their pyramidal forms. The fragments of 

 rock, both angular and rolled, that cover the valley, were found to be of green clay 

 schist, the same metamorphic greenstone seen yesterday, and a greenish sandstone. 



In the forenoon we reached Urga, also called Ivuren, the residence of a living 

 Buddha. 



Dec. 14. Left Urga for Kiachta, Avhich place we reached on the 21st December. 

 The country between these places was covered with snow, concealing its geological 

 character. Our road lay through the hills to the eastward from the Orklion river, 

 crossing its tributaries, the Kara Gol and the Iro Gol. 



Through the first two-thirds of the distance the few outcrops seen were of rocks 

 similar to those seen near Urga; at Iro Gol I found chloritic granite. 



A great steppe deposit, apparently of loose argillaceous sand, fills the valleys, 

 and, extending over the lower parts of the crests of the ridges, leaves the higher 

 peaks isolated Hke the islands of an archipelago. This is part of a very extensive 

 deposit which, from its position here, must be continuous through all the lower 

 course of the Orkhon. It would seem to be the same deposit that forms the broad 

 steppe south of Kiachta, and is visible, I think, in the terraces of the Selenga as 

 far as Lake Baikal, and in the tables on either side of the Angara at Irkutsk. 



The barometrical measurements of the Russian Academicians, MM. Fuss and 

 V. Bunge, have shown that that part of the continent which they crossed, between 

 the Great Wall of China and the Siberian frontier, south of Lake Baikal, is an 

 elevated plateau, bounded on the N. W. and S. E. by mountain ranges from 5000 

 to 10,000 feet high, from the sides of which the table-land falls gently toward a 

 broad level region in the centre, the mean height of which is not more than 2400 

 feet. 



The skeleton of the plateau is thus a great geoclinal valley, trending nearly N. E., 

 the basis of which, so far as obsen^ed, is formed by granitic rocks, and metamorphic 

 strata, probably of Paleozoic origin, and the inequalities of Avhich have been nearly 

 filled up with more recent formations. Of these latter we can, at present, recognize 

 only three, viz: — 



1. The great development of lava along the southern edge. 



2. The steppe deposit including the Gobi sandstone. 



3. The deposits of loam, mentioned in the preceding pages as covering in places 

 the steppe deposit. 



The lava formation is apparently the oldest of the three. We have seen, in a 

 former chapter, how a part, at least, of the southeastern edge of the table-land owes 

 its level surface solely to the great thickness of the volcanic rocks, which have thus 

 been able to fill up the hollows between the ridges of granite and metamorphic 

 rocks. The profile, constructed from the measurements^ of MM. Fuss and v. Bunge, 

 seems to indicate the existence of a terrace from 3000 to 4000 feet high and about 

 150 miles broad, that forms the S. E. border of the plateau. It is not improbable 

 that this terrace is due, in great part, to extensive lava flows. 



The volcanic rocks of Lake Baikal and of the region to the east, the occurrence 



' Ritter. 



