512 THE TERTIARY PERIOD 



strata, chiefly limestones, are also quite extensively developed in 

 the West Indian islands. 



In California an elevation at the end of the Eocene, or early in 

 the Miocene, had shifted the shore-line far to the west. The late 

 Cretaceous peneplain of the Sierra foothills had been elevated 

 and carved into ridges. In the lower stream courses and the 

 deepest parts of the valleys some heavy gravels had accumulated 

 (the deep Auriferous Gravels). In the Upper Miocene the sea 

 again advanced, raising the shore-line 420 feet above the present 

 sea-level, depositing sediments (the lone stage) along the foothills 

 of the Sierra. In the stream valleys very thick masses of gravel 

 were accumulated (the bench Auriferous Gravels). After the 

 deposition of the gravels came a time of great volcanic activity in 

 the Sierra, first, of rhyolite flows accompanied by sheets of tuff, 

 and, after an interval, of andesite tuffs and breccias, which poured 

 down the valleys as immense torrents of mud. Marine Miocene 

 beds are found on both sides of the Coast Range, which then 

 formed a chain of reefs and islands. Into the northern part of 

 the Sacramento valley, the sea did not extend, this portion being 

 occupied by a lake which is supposed to be Miocene. The coast 

 of Washington and Oregon, including much of the Coast Range 

 in the latter state, was covered by the sea, which extended up the 

 valley of the Columbia and that of its southern tributary, the 

 Willamette. Puget Sound was broader than now, and more widely 

 connected with the Pacific ; indeed, the whole peninsula between 

 the sound and the ocean, with the Olympic Range, may have been 

 submerged. The sea also extended over parts of the British 

 Columbian coast. Alaska was depressed in the early Miocene, 

 especially toward the north, and the valley of the Yukon was in- 

 vaded by the ocean; probably the eastern part of Bering's Sea, 

 and much of the lowlands of western Alaska, were covered with 

 shallow water. 



In the interior region are extensive fresh-water deposits of 

 Miocene date. The oldest of these form the John Day stage of 

 eastern Oregon, which spreads between the Cascade and Blue 

 Mountains, and southward may extend into Nevada. The thick- 



