632 IDAHO MINING DISTK1CT8. 



now deeply covered below later formations. If we should venture 

 tentatively to go back one step further, it might be suggested that 

 the uplifted surface of the Boise Mountains is probably the result of 

 a far older erosion, of early Tertiary or Cretaceous age, which planed 

 down a more ancient range to gentler outlines, or to a peneplain. 



THE PAYETTE FORMATION. 



During the earlier part of the Neocene (Miocene) a large fresh- 

 water lake occupied at least the lower part of the Snake River Val- 

 ley, and its sediments are now prominent features of the region. 

 For these lake beds the name Payette formation is proposed, and 

 their age is determined as upper Miocene. This formation is prob- 

 abty not the same as Cope's Idaho formation, to \fhich a Pliocene age 

 was assigned and which appears to be connected with the later basalt 

 flows. 1 



The extent of the formation is shown in PI. LXXXVII, from which 

 it is seen that it lies in front of the Boise Mountains and occupies the 

 whole lowei* part of the ridge between the Boise and the Payette. It 

 extends over large areas to the north of the Payette, along the flood 

 plains of the Snake River, and is seen to occupy vast areas in Oregon 

 between the mouth of the Owyhee River and Weiser, where the 

 Snake River Canyon begins. On both sides of the lower Snake River 

 the bluffs of the Payette formation attain a height of over 800 feet. 

 In the Payette Valley south of Emmett the sharply defined bluff of 

 Payette beds rises 600 feet above the alluvium. Smaller masses, 

 detached by erosion or uplifts, lie in the intermontane valleys as far 

 east as the Idaho Basin. 



Along the Boise Mountains the Payette beds rest against the irreg- 

 ularly eroded and sharply sloping surface of the granite, and the top 

 stratum attains a height of 4,100 feet. A total thickness of 1,000 feet 

 is exposed near Boise, and wells bored show several hundred feet of 

 similar strata below the surface. Over the larger part of its extent 

 the formation lies nearly horizontal or dips only a few degrees. Near 

 the mountains dips of 8 to 10, generally westward, are noted, and the 

 smaller detached masses in the intermontane valleys are still more 

 disturbed, generally dipping westward at angles up to 50. This is 

 particularly marked in the long arm of sediments of the Payette for- 

 mation filling the valleys of Horseshoe Bend and Jerusalem, on the 

 Payette. 



As might be expected from the character of the land mass from 

 which the sediments were obtained, the latter consist chiefly of granitic, 

 light-colored sands, locally cemented by hot-spring deposits to hard 



1 From the results of more extended surveys during the summer of 1897, it has become evident 

 that the Payette and Idaho formations represent two successive stages of the lake, the often 

 deformed shore-line of the former being found at elevations of from 4,200 to 5,000 feet, and those 

 of the latter at a maximum elevation of 3,000 feet. To separate the deposits of the two forma- 

 tions is not always easy. 



