408 HISTORICAL GEOLOGY 



the period sediments had been accumulating along the margin 

 of the ocean, which at that time spread eastward as far as 

 Nevada. Probably deposition had been going on along this 

 coast through several of the preceding periods also. By the 

 close of the Jurassic period the result was a very thick body 

 of marine sediments, chiefly shales and sandstones, which 

 had been laid down in shallow water, and, as the general 

 fineness of the material testifies, near a coast which was not 

 rugged. 



Crumpling on the Pacific border. At the end of the 

 period this long cycle of deposition was interrupted by one of 

 those intense crumplings of the earth's crust, which at inter- 

 vals throughout geologic history have disturbed first one 

 locality and then another. By lateral compression, the driv- 

 ing force of which seems to have originated in the deep basin 

 of the adjacent Pacific Ocean, the shales and sandstones were 

 closely folded so that the beds now stand on edge. At about 

 the same time, great quantities of igneous rock, especially 

 granite, welled up into the folded mass, and solidified in the 

 form of stocks and huge batholiths. On the borders of these 

 intrusions, the sedimentary beds were changed into schists 

 and other metamorphic rocks. Even at a distance from the 

 igneous masses, the intense pressure exerted was sufficient to 

 convert the deeply buried shales into hard slates, and, in 

 some cases, to metamorphose the rocks even more severely. 

 The result of this series of disturbances was doubtless a wrin- 

 kling and cracking of the surface of the land parallel to the 

 Pacific through California and probably as far north as 

 Alaska. Mountain ranges were raised on the site of what 

 had previously been a shallow sea. Even as they were ele- 

 vated, these mountains were being gradually dissected by 

 running water and the other agencies of degradation, just as 

 the rising Sierras to-day are being sculptured. In their youth, 

 we may well suppose that these early ancestors of the present 

 Sierra and other Pacific ranges were lofty and rugged moun- 

 tains. They have since, however, been worn down and have 



