CHINA, MONGOLIA, AND JAPAN. 27 



Leaving this plain, we again rise to the table-land, and following, for six or seven 

 miles, its abrupt edge we come again to a sudden descent by which we leave it and 

 enter upon a rolling country. The plateau wall makes here a great bend, trending 

 away to the northwest. 



The country over which our road now lies is a rolling plateau formed by a broad 

 swell, or ridge, of the granitic and schistose rocks, from which the volcanic plateau 

 covering has been eroded. On it are the sources of another tributary of the Yang Ho. 



The rocks are granite, syenite, and crystalline metamorphic schists. 



This bay-shaped indentation of the southern edge of the plateau is about 15 or 20 

 miles broad; it is drained in part by a valley descending toward the southwest, and 

 is surrounded on the east, vilest, and north by the wall of the higher plateau. The 

 northern portion of this bay forms a depression that is only partially drained, and 

 which at times is evidently a marshy region, while it contains at all seasons three 

 small lakes — Gurban Noor. In April the country about these lakes was covered 

 with scattered tufts of grass, between which the dry clayey surface was white with 

 an efflorescence of soda, and the borders of the lakes also were incrusted with a 

 dazzling layer of the same salt. 



About two miles west of the Mongolian camp of Gurban Noor, the higher table- 

 land again begins, but with a somewhat different character. Rising to the top of a 

 granite ridge, we descend a little on the west into a plateau-valley. On either side 

 and before us are everywhere the same flat-topped hills we have ' seen forming the 

 table-land, but they are only the remnants of a volcanic covering insignificant in 

 thickness compared with that we have seen farther east. The valleys have every- 

 where cut through this covering and into the granito-schistose foundation. 



Our road now lies through a succession of circular and oblong meadow-valleys, 

 connected by narrow outlets, thus forming one valley-course, and containing a small 

 brook, the Hoyurtoloho Gol, which flows S. E. The meadow enlargements are evi- 

 dently the beds of small lakes filled with the detritus of the surrounding volcanic 

 and granitic rocks. 



Following this valley in a general S. W. direction from the Mongol camp, Hoyur- 

 toloho Gol, we descend through a narrow defile in chloritic granite, into another bay 

 cut out of the plateau, and open to the S. E., where the drainage finds an exit 

 through the valley of the Si Ho, another tributary of the Yang Ho. 



Soon after leaving the gorge, by which we have descended, the road crosses a lava 

 stream one or two thousand feet broad, and from sixty to eighty feet thick, which 

 crosses the valley, and is cut through by the rivulet. In this section it shows 

 columnar structure, and is in places porous and amygdaloidal. A mountain form- 

 ing apparently a detached portion of the neighboring plateau, and having the ap- 

 pearance of a half-destroyed crater, seems to be the origin of the stream. The 

 eruption causing this occurrence must have been subsequent to the erosion of this 

 part of the plateau, and was probably subaerial. The locality is interesting as 

 being the only one in which I noticed traces of true volcanic action more recent 

 than that to which the volcanic formation of Southern Mongolia owes its origin. 

 Crossing the valley of the Si Ho, which leaves this bay-shaped depression at the 



