SINO-MONGOLIAN FRONTIER 



colour. This colour on the exposed surface is 

 changed, through weathering, to a dark brown. 

 Typical crystals of felspar occur throughout the 

 mass. In the specimens gathered the crystals 

 had undergone considerable decomposition. 



At a place called Hsi-tien-tzu the valley takes 

 a bend eastward, at right angles to its original 

 course, while a wide but waterless valley enters 

 it from the north. This is called Ching-kou, 

 while the main valley, which has a good per- 

 manent stream, is now called Ch'ing Ho. 



The trachyte continues all the way to Kao- 

 chia-yin-tzii, just beyond which place it ends. 

 Here it is of a lighter colour, the weathered sur- 

 face being of a bright brick red, while the new 

 fracture shows the rock itself to be of a light 

 mauve-grey. The white crystals of felspar are 

 also much larger and more noticeable than in the 

 outcrops nearer Kalgan. Where they are exposed 

 at the surface, these crystals rapidly disintegrate 

 and are washed away, leaving the surface rock full 

 of holes, which gives it something the appearance of 

 a rock charged with air-bubbles. 



Beyond Kao-chia-yin-tzu, still working east- 

 ward, we found metamorphic (chiefly gneiss) 

 rocks, alternating with plutonic (granite), the 

 former predominating. 



Both the metamorphic and plutonic rocks are 

 seamed with stringers, leads and ledges of quartz 

 (probably auriferous). 



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