LINDOKEN.] PRE-TERTIARY ROCKS. 631 



bodies of schist appearing in connection with it. It weathers easily 

 and crumbles to a coarse sand which largely covers the hillsides. 

 Only in the higher mountains and along the bottom of the canyons 

 are satisfactory exposures found. The age of this granite, which is 

 clearly of igneous and intrusive origin, is an open question. Messrs. 

 Becker 1 and Eldridge 2 assign to it provisionally an Archean age, but 

 a thorough study of its contact with surrounding formations is neces- 

 sary before its age can be determined. The granite is in many places 

 traversed by shear planes, giving it a jointed or sheeted structure, 

 and often these planes coincide with the direction of the fissures on 

 which mineral veins occur. It is probable that these two features 

 are of the same and contemporaneous origin. Nearly all of the pri- 

 mary mineral deposits are contained in the granite or allied porphy- 

 ries. By far most of them have a direction ranging from E.-W. to 

 ENE.-WSW., and dip to the south at angles from 40 to 85 from the 

 horizontal. While it is probable that all of them belong to the same 

 period of formation, there are few definite clews to their age. It is 

 likely, however, that they are post-Carboniferous, and it is certain 

 that they antedate the Miocene lake deposits. A Cretaceous or early 

 Tertiary age may provisionally be assigned to them. The mode of 

 their occurrence indicates beyond doubt an origin by deposition from 

 mineral waters, probably ascending hot springs: A slight recurrence 

 of the vein-forming activity occurred after the Neocene period. 



Before the beginning of the Neocene the chief features of the topog- 

 raphy were outlined the broad uplift of the Boise Mountains and the 

 depression of Snake River Valley. The latter is not unlikely a sunken 

 area separated by old fault lines from the mountains to the north. 

 At that time the basalt flows and the lake beds did not exist, but the 

 drainage of the Boise, and probably also of the upper Payette River, 

 was outlined in practically its present form. The granitic range pre- 

 sented a bold scarp facing the valley, and the canyon of the Boise 

 R^ver was, at its debouchure from the mountains, cut to practically 

 the same depth which it has at present. It had not, of course, cut 

 back so far toward the Sawtooth Range as at present, and many 

 features of the drainage, notably in the Idaho Basin, were different 

 from those existing now. As substantiating this it will be shown that 

 the Miocene lake beds fill the old canyon at the gate of the moun- 

 tains, 10 miles southeast of Boise, and that in front of it lie enormous 

 masses of coarse Neocene gravel and conglomerate. Thus the time 

 immediately preceding that from which the first records date was one, 

 first, of uplift and subsidence, during which the rough features were 

 blocked out, and second, one of long-continued erosion, during which 

 the Boise Mountains were dissected and the debris from the excavated 

 canyons deposited in the basin of the Snake River Valley, where it is 



1 Tenth Census, Precious Metals, p. 54. 



2 Loc. cit. 



