GEOLOGIC HISTORY OF CHINA BLACKWELDER. 387 



we may picture China as a shallow sea bottom rising very gradually 

 to a marshy coastal plain on the east. During the long intervening 

 ages the accumulation of sediments upon the sea bottom had formed 

 successive layers of limestone, shale, and sandstone^ which eventually 

 reached a thickness of 5,000-10,000 feet. 



This condition did not hold without end, for eventually^ strong 

 compressive forces engendered in the underlying body of the earth 

 squeezed the superficial rocks into folds, and thus bulged the surface 

 high above sea level in the region so affected. By the prompt attack 

 of streams, winds, glaciers, and the other agencies which are inces- 

 santly sculpturing the surface of the earth, these eleyated districts 

 were, even while rising, carved into rugged mountains and deep val- 

 leys, so that the original folds were greatly disfigured, even before 

 the compressive forces ceased to operate. 



It is a fact generally recognized among geologists that in terms of 

 geologic time such episodes of compression and folding are short 

 lived. They are soon followed by much longer periods, during which 

 the internal forces of the earth are quiescent but in which the erosive 

 agencies have free play. If any land remains indefinitely above sea' 

 level and is not disturbed by movements from below, the mountains 

 and hills will eventually be worn away and there will be left only a 

 broad, almost featureless, plain. It is believed that China, in conse- 

 quence of such a period of quiescence,^ was reduced to a lowland from 

 which almost all of the preexisting mountains had been removed. In 

 this condition it probably remained for more than one geologic 

 period, and the western part may even have been submerged beneath 

 the sea which at that time covered northern India and part of Tibet. 

 In that sea were deposited the thick beds of limestone which are now 

 found in some of the western mountain ridges. 



Again, in the Miocene period the forces of distortion withm the 

 earth accumulated to such strength that they were able to repeat the 

 mashing and folding, but this time the area affected lay farther to the 

 west and south. At the same time, or perhaps earlier, the eastern 

 part of China was cracked in various directions ; and the intervening 

 blocks, settling somewhat unevenly upon their bases, left a group of 

 escarpments and depressions comparable to those now to be found in 

 western Nevada and southern Oregon, As before, the work of erosion 

 and the leveling of the surface was at once accelerated, so that even 

 before the deformation had spent itself the blocks were deeply 

 scarred. It is uncertain how far this period of erosion succeeded in 

 reducing China to base level. The consummation may have been 

 prevented by gentle warpings of the surface, rising very slowly here 

 and sinking there. When compared with the great breadth of the 



1 Jurassic period, " Cretaceous and Eocene periods, 



