636 IDAHO MINING DISTRICTS. 



uplifts, the detailed character of which will be discussed in the text 

 accompanying the Boise folio of the Geologic Atlas of the United 

 States. Important movements also took place along the Boise Ridge, 

 which separates the Idaho Basin from the Payette River, and it is 

 probable that it has undergone an extensive uplift. Over the larger 

 part of the area no orographic movements have affected the beds. 



LATE NEOCENE BASALTS. 



When the lake had been partly drained, vast basaltic eruptions 

 began, and in time, intercalated with lake beds, filled the whole of the 

 upper Snake River Valley from the base of the Tetons, near the Wyo- 

 ming line, to a point near the confluence of the Boise and the Snake. 

 Between this point and Weiser no basalts are seen. The basalt flows 

 lie horizontal, filling the plains and the modern canyons. They are 

 also distinguished by their fresh character, black color, and columnar 

 structure. The aggregate thickness probably never exceeds 1,000 

 feet, and is ordinarily much less, individual flows being rarely over 

 100 feet thick. 



As already indicated, the basalts were erupted from a great num- 

 ber of inconspicuous craters, both in the plains and in the adjoining 

 mountains. Their fluidity was remarkable, continuous flows of 50 

 miles or more being noted. Where the Boise River emerges from the 

 mountains the exposures are exceptionally good. There are three or 

 four flows, the principal ones coming down from the South Fork of 

 the Boise. The deepest flow of comparatively small volume is prob- 

 ably the oldest, and lies but a few feet above the present bed of .the 

 river. Deep river gravels accumulated on this flow, and, soon after- 

 ward, two later flows came down the Boise River and filled the canyon 

 near the mouth to a depth of 300 feet. Beyond the mouth the basalt 

 spreads out, and its surface rapidly sinks westward. Still another 

 basalt flow, about 75 feet thick, came down from Moore Creek and 

 joined the large ones at the mouth of the main river. Above the 

 source of this basalt the damming resulted in terraces and bench 

 gravels now lining the upper valleys of Moore Creek and Grimes 

 Creek, described in Chapter III. 



POST-BASALTIC EROSION. 



If the epoch of the basaltic flows be placed at the very close of the 

 Neocene, the events that have taken place since then must be referred 

 to the Pleistocene. Among these are the erosion of the canyons of 

 Snake River and its tributaries to a depth of from 200 to 700 feet and 

 the deposition of extensive flood plains and terraces along the lower 

 Snake, Boise, and Payette. The Boise River has, in Pleistocene times, 

 cut through the 300 feet of basalt accumulated at the mouth of its 

 canyon, and thus laboriously regained the same stage it occupied 

 before the beginning of the Payette epoch. The direction of the chan- 

 nel has gradually changed. During the Payette epoch it had a nearly 



