630 IDAHO MINING DISTRICTS. 



belonging to the former, and the divide is in process of migration north- 

 eastward. The whole region may be regarded as an uplifted sloping 

 plateau deeply dissected by a drainage system, whose origin evidently 

 antedates the Miocene period. Smaller individual ranges occur in a 

 few places, as in the Boise Ridge, rising to elevations of 7,500 feet 

 and extending due north, dividing the Idaho Basin from the waters 

 of the Payette. This range also crosses the South Fork of the Payette 

 and continues for some distance north of it. Within this mass of 

 mountains several depressions or basins with gentler slopes also exist, 

 such as the Idaho Basin, the Dead wood Basin, and Smiths Prairie, 

 which have been created or emphasized by more recent orographic 

 movements. Evidences of glacial topography occur only near the 

 Sawtooth and Trinity mountains. 1 The lower area here specially 

 described has never been covered by ice. 



GEOLOGICAL HISTORY. 



The vicinity of Boise River, where it debouches from the mountains, 

 proved to be an exceptionally fortunate location for the study of the 

 geological history of this part of the Snake River drainage, for the 

 record left by the river of successive geological events back to a cer- 

 tain date is remarkably clear and easy to read. 



PRE-TERTIARY. 



The oldest rock exposed is the granite of the Boise Mountains. This 

 forms an extremely large area, embracing, so far as known, the whole 

 of the upper drainage of the Boise and Payette rivers and extending 

 northeastward beyond the Sawtooth Mountains and eastward as far 

 as Wood River, where it is adjoined by sedimentary rocks of probr 

 ably Carboniferous age. 2 This rock is largely a typical, coarse gran- 

 ite of gray or yellowish-gray color, consisting of orthoclase in often 

 large crystals, plagioclase, quartz, biotite, and sometimes muscovite. 

 Pegmatite dikes are common in many places. Locally the granite 

 contains hornblende, and is by gradual transition connected with 

 intermediate rocks standing between granite and diorite, and also, 

 though more rarely, with diorites. Narrow dikes of light-colored 

 granite-porphyry and dark lamprophyric dike rocks, chiefly minettes, 

 are abundant and present a great variety of structural types. A belt 

 characterized by dikes of coarse quartz-diorite, porphyrites, and occa- 

 sional occurrences of gabbro and diabase extends, with one short inter- 

 ruption, from the vicinity of Wilson Peak, east of the Idaho Basin, by 

 Quartzburg, to the Willow Creek mining district. All of these dikes 

 are probably connected with the granite eruption that is, they were 

 intruded shortly after the consolidation of the granite. Within the 

 area described the granite is remarkably unaltered and massive, no 



1 George H. Eldridge, Sixteenth Ann. Kept. U. S. Geol. Survey, Part II, 1895, p. :*>:!. 



2 George H. Eldridge, loc. cit. 



