CHINA, MONGOLIA, AND JAPAN. 



23 



another enlargement of the Yang Ho valley, and are also lake terrace deposits. The 

 road lies over this lake bed till about ten miles N. E. of the city of Siuenhwa (fu), 

 where a spur extends westward from the mountains. This spur consists of a double 

 ridge, with an intervening longitudinal depression, the southernmost portion being 

 formed by beds, highly inclined to N. and trending E. W., of quartzite, red argil- 

 laceous sandstone, and a compact Avhite rock, apparently an altered argillite. These 

 beds, which seem to be the equivalent of the great limestone formation, will be 

 referred to again in discussing the Hwaingan strata. , 



The northern part of the double ridge is a remarkable porphyry, which has either 

 traversed or overlies the last mentioned beds. This rock may be called the Kalgan' 

 porphyry, as it is extensively developed around that city, although it occurs also in 

 the hills of the Gobi desert. It belongs to the trachytic series. 



On the southern flank of this spur the lake deposit rises rapidly toward the hills, 

 and the firm loam, of which it here consists, is cut into by deep gullies. In one of 

 these places a section is exposed of horizontal beds, apparently the tufas of the 



a. Terrace loam. 6. White tufa. c. Red tufaceons sandstone. 



Kalgan porphyry. The effects of an erosion previous to the deposition of the lake 

 loam are visible. 



We shall find similar tufaceons deposits intimately associated with the Kalgan 

 porphyry near that town. 



From the spur we have been examining we follow the road over the lake deposit, 

 to Kalgan, or Changkiakau. High and rugged hills of the trachytic porphyry 

 inclose the valley on the east, while to the north lies a higher range of mountains, 

 which, as it forms a geographical as well as political boundary, and represents 

 approximately the line of the Great Wall, we may call the Barrier range. 



a. "White and red tufas. 6. Kalgan porphyry, c. Tower of the Great Wall. 



At Kalgan this range is traversed by a gorge, with vertical walls, through which 

 a small stream finds its way to the Yang Ho from the edge of the Mongolian plateau. 



• The Russian name for Changkiakau, an important market town and gate of the Great Wall. 



