648 IDAHO MINING DISTRICTS. 



MINERAL DEPOSITS OF POST-NEOCENE AGE. 



As has been explained before, there is every reason to believe that 

 all of the more important ore deposits antedate the Payette formation, 

 or that, in other words, they are pre-Miocene. But there is also some 

 evidence of a later period of ore deposition, although of less impor- 

 tance, which occurred after the early Neocene eruptions, and which 

 may be going on in depth even at the present time by means of the 

 hot ascending spring waters found at several places in this region. 



The Neocene rhyolite area occurring on the Idaho City road 3 miles 

 from Boise is somewhat altered in places and is stated, on reliable 

 authority, to contain $1 per ton in silver and a trace of gold. A 

 Neocene sandstone with veins of opal, occurring near this area along 

 the road, was assayed and found to contain 0.50 ounce of silver per 

 ton. The knob of partly altered augite-andesite near the penitentiary 

 at Boise was assayed and found to contain 0.05 ounce of gold and 

 0.50 ounce of silver per ton, a total value of $1.38. 



The Neocene basalt at the mouth of the canyon of Jackass Creek, 

 Jerusalem Valley, contains a quartz vein which is stated to assay 

 about $4 in gold and silver. 



