72 ASIA 



themselves gigantic chasms and eaten their way into the 

 heart of the plateau, partly clearing away the glacial 

 wastes. 



Tibet is not entirely rainless, but receives a large pro- 

 portion of its scanty water in the form of snow. The 

 north-western half of its area appears to be drier than 

 the southern and eastern portions. It is open, by reason 

 of its altitude, to an excessive evaporation and radiation. 

 The climate is extremely cold, but relieved in summer by 

 some warm days, though terrific snow-gales sweep across 

 its surface in winter. The soil, which consists mostly of 

 glacial and alluvial rubble, shingle and gravel, naturally 

 adds by its excessive porosity to the climatic dryness ; 

 only the fine silts and clays along the rivers and around 

 the lakes possess a natural fertility. Tibet may be 

 described, on the whole, as a cold desert. 



Over vast expanses the rubble and gravel may be entirely 

 plantless. The finer soils support a thin sprinkling of 

 tufted grasses and a few other rosette herbs, but trees and 

 shrubs are absent. The bands of alluvia along the rivers 

 often permit of the development of short pastures, and 

 even mats of dwarf grasses of an arctic-alpine type. 

 Special or strange plant-forms are rare, and there does 

 not appear to be the wealth of tuberous and bulbous 

 forms which characterize the South- American punas: 

 even the large cushion plants do not seem to be recorded 

 as striking features. Keed and rush-swamps also occur 

 near the rivers and lakes, but peat-bogs are not 

 reported. Most plants are dwarf and crawling, or in 

 spread rosettes, and forms above one foot in height are 

 rare. Not all parts of Tibet, however, are equally barren : 

 going eastward,the fertility increases and grazing grounds 

 are better. At the head- waters of the various Chinese 

 rivers, the alpine meadows are extensive and generous; 



