696 



IDAHO MINING DISTRICTS. 



upper Rabbit Creek, draining into Boise River 8 miles east of 

 Idaho City. 



Fineness of quartz gold. 



a Average. 

 THE GEOLOGICAL HISTORY OF THE IDAHO BASIN. 



The succession of geological events to which the existence of the 

 basin and of the gold-bearing gravels is due is neither simple nor easy 

 to decipher. In a large degree this is owing to the very monotonous 

 structure of the bed-rock series, which gives few clews, except those 

 indicated by the topography, to the character of the movements that 

 have taken place, for it soon becomes apparent, during a study of the 

 district, that erosion alone, unaided by orographic movements, can 

 not have produced this peculiar depression situated on the divide 

 between two main rivers. 



The doubtful age of the granite, which alone constitutes nearly the 

 whole pre-Tertiary series in the basin, has already been alluded to. 

 It has further been stated that a surface laid through the ridge lines 

 of the Boise Mountains in general probably forms part of an old pre- 

 Tertiary peneplain or land mass planed down by erosion; and, still 

 further, that the erosion succeeding the uplift which differentiated 

 the Boise Mountains and the Snake River plains had, prior to the 

 lake period, cut far into this uplifted surface, so far, indeed, that 

 the Boise Canyon at its mouth was cut to its present depth at the 

 beginning of the Neocene. All this does not account for the depres- 

 sion of the basin, which lies much below the general level of that 

 surface. It seems probable that the present upper valleys of Grimes 

 and Moore creeks have been excavated by erosion, but this again 

 does not account for the basin as a whole. In it the river valleys 

 are separated by low ridges, the summits of which form, if extended, 

 an undulating surface considerably above the creeks, it is true, but 

 still much below the general surface of the surrounding country, as 

 is well shown by PI. XCI. The extent of this surface determined 

 the existence of the basin in the first place, and as a probable work- 

 ing hypothesis to account for this it may be assumed that this earlier 



