LINDGREN.] GEOLOGICAL HISTORY OP THE IDAHO BASIN. 697 



basin was caused by a sinking of a portion of the Boise Mountains 

 along curved fault lines between the Boise Ridge on the west, Wilson 

 Peak and other elevations on the east, and the Thorn Creek Hills on 

 the south. The age of the quartz veins in the basin can not be defi- 

 nitely indicated, but is most likely Cretaceous or Eocene. They 

 were certainly formed before the deposition of the Payette lake beds, 

 and it is to be expected that the rivers and creeks of the pre-Payette 

 period of erosion in the basin carried detrital gold derived from 

 these veins. These stream gravels are now either completely eroded 

 or buried below the lake beds at Idaho City. As has been ex- 

 plained in the detailed description, there is slight chance of finding 

 them by boring, and still slighter chance of mining them profitably 

 if found. 



The surface of the Payette lake attained a height above the present 

 sea level of 4,200 feet at the mouth of the Boise Canyon. If the eleva- 

 tions and the topography were then the same as at the present time, 

 the lake would have reached up as far as Centerville in Grimes Creek, 

 and 4 miles above Idaho City in Moore Creek. It is almost certain, 

 however, that the relative elevations are not the same, for near Idaho 

 City we find the Payette lake beds at 4,400 feet, and there forming 

 part of a smaller area which has settled down between parallel fault 

 lines just how much is not known. From this it appears that the 

 basin has increased its elevation somewhat relatively to the country 

 at the mouth of the Boise Canyon. High up near the divide, on Muddy 

 Creek, a small remnant of inclined lake beds occurs, but it is perhaps 

 a small local accumulation. At any rate it is certain that the Payette 

 lake covered the lower part of the basin in early Neocene times. 



The lake beds were rapidly accumulated in the bay then occupying 

 the basin, and as no concentration of the material took place, it was 

 natural that their content of native gold should be very slight. 



The raising of the base level to the present elevation of over 4,000 

 feet would naturally produce extensive accumulations of gravel in 

 the creeks draining to the lake. It is probable, indeed, that at this 

 time of maximum lake extension the lower valleys opening into the 

 basin were choked with gravel; and as an evidence of this may be 

 cited the occurrence of auriferous river gravel on the summit of a 

 ridge in the Thorn Creek drainage, at an elevation of 4,500 feet. 

 Vast eruptions of basic lavas took place on the summit and western 

 side of the Boise Ridge, and to less extent in the basin. All of these 

 earlier lavas probably flowed out during the deposition of the lake 

 beds. As the lake receded stream courses were established over its 

 deposits, and the streams which headed near quartz veins began to 

 carry down their precious load and concentrate the gold on the bed 

 rock. These fluviatile deposits, which were formed very shortly after 

 the Payette lake beds, or perhaps in part contemporaneously with 

 them in valleys draining into the lake, have been described as "older 



