634 IDAHO MINING DISTRICTS. 



From these data Professor Knowlton draws the conclusion that the 

 age is Upper Miocene, contemporaneous with the flora of the aurifer- 

 ous gravels and the lone formation of California, the Lamar flora 

 of the Yellowstone National Park, and the John Day formation of 

 Oregon. The paleobotanical evidence confirms the conclusion, confi- 

 dently drawn from the field work, that all these smaller detached 

 masses of lake beds are of practically the same age. 



During the time of the maximum extension of the Payette Lake its 

 surface stood at the present elevation of 4,200 feet. Its deposits, over 

 1,000 feet thick near the shore, rested against the abrupt slope of the 

 Boise Mountains, and filled the old canyon of the Boise to the same 

 depth. The canyon must have formed a fiord, the branches of which 

 reached as far back as the Idaho Basin, and in which vast quantities 

 of gravel and sand accumulated. Isolated occurrences of well- washed 

 gravel on the summit of high ridges in the lower Moore Creek drain- 

 age, at elevations of 4,500 feet, confirm the above conclusions. The 

 data are not at present sufficient to determine the extent of the Payette 

 Lake, though it is probable that it was confined to the Snake River 

 Valley, inclosed on the west by the Blue Mountains and on the east 

 by the divide toward the Salmon River. 



EARLY NEOCENE VOLCANIC ACTIVITY. 



Near the base of the Payette formation sheets of rhyolite and 

 rhyolite-tuff occur, but this eruption was of limited extent. The 

 best exposures are found near Boise and in the Willow Creek mining 

 district. After the rhyolitic eruptions there occurred enormous out- 

 pourings of basaltic lavas, distinctly different from and earlier than 

 the Snake River basalts. The rocks are in part normal basalts, but 

 have usually a somewhat andesitic habit. In large areas the outcrops 

 generally have a reddish-brown color, distinct from the somber black 

 of the later flows. These early Neocene eruptives are to some extent 

 represented near Boise, but become more abundant northward. Many 

 exposures are seen along the lower Payette from Marsh post-office to 

 Horseshoe Bend and Jerusalem Valley, and the conspicuous sharp 

 ridge of Squaw Butte, rising just north of the Payette above the lake 

 beds to an elevation of 5,800 feet, is composed entirely of these older 

 effusive rocks. Squaw Butte is well visible from the railroad near 

 Nampa and Caldwell as a rugged, reddish-brown peak, contrasting 

 with the white lake beds. The evidence shows that these flows were 

 contemporaneous with the deposition of the Payette beds, and are 

 underlain and covered by sandstones. In some places they break 

 through the lower lake beds and metamorphose them. The vents 

 were located chiefly along the margin of the old lake. At Jerusalem 

 (see PI. LXXXIX) arid in Squaw Butte the whole volcanic series in 

 the latter place consisting of countless flows, attaining a thickness of 

 several thousand feet has been disturbed and uplifted, and now 



