CHANGE-PKODUCING AGENCIES 653 



A great mountain range, several hundred miles long, forms the 

 sharply defined southern edge of the desert plains of eastern central 

 Asia. It rises everywhere abruptly from this plain to a height of from 

 5,000 to 10,000 feet, and its height is sufficiently great to cause it to 

 receive abundant precipitation and a heavy covering of winter snows. 



Within this mountain system the trunk valleys, after following a 

 longitudinal course, turn sharply and, cutting through the border range 

 and piedmont hills, debouch their waters onto the plains. 



The mountain masses, lacking the protection of a heavy forest growth, 

 are subjected to rapid disintegration and decay, and the resulting detri- 

 tus is carried by the torrential rivers down to the plains. 



In a coastal region these waters would flow onward to the ocean and 

 the silts they had brought from the mountain would ultimately complete 

 the same course, to be deposited at the mouth of the river, to form there 

 a submarine delta ; but in an arid "central" region, such as is Turkestan, 

 the conditions are different. The precipitation is confined to the moun- 

 tains, and on leaving these the rivers immediately spread out in a region 

 of rapid evaporation, where there is no compensating rainfall, for the 

 valley ends at the mouth of the mountain gorge. 



Thus all the coarse and fine materials brought by the torrential rivers 

 from far and near in the mountains are deposited within a zone along 

 the edge of the plain at the base of the Kopet range. The rock-mass of 

 the mountains is therefore being continually removed and loaded onto 

 this long zone. 



Now two connected phenomena result from this process: on the one 

 hand, the zone of deposition is continually and proportionately sinking 

 under the increasing load, and on the other hand the mountains are con- 

 tinually rising to maintain their height. 



The strain established in the rigid crust between the sinking zone of 

 deposition and the rising mountain range finds relief in the development 

 of fractures along the range, as well as others which permit a differential 

 uplifting of great block-masses. The evidence of this compensatory 

 maintenance of hydrostatic equilibrium is strikingly recorded both in 

 the Kopet range and in the zone of deposition. All along the range the 

 lines of fracturing are visible on a large scale in well developed faultings, 

 and the border of the alluvial plain is bent sharply upward, having been 

 dragged up by the rising mountains. 



Deep longitudinal valleys are carved by erosion along the lines of 

 weakness offered by these fault-planes. On the mountainward side of 

 the valley rise the older rocks of the range, while on the other is a steep 



LIX — Bull. Geol. Soc. Am., Vol. 17, 1905 



