CHAPTER IV. 



THE IDAHO BASIN (CONTINUED). 



THE PRE-TERTIARY ROCKS. 

 GRANITE. 



The Idaho Basin forms a part of the great granite area of the Boise 

 and Payette drainage, and the pre-Tertiary rocks consist exclusively 

 of granite, together with a number of porphyritic dikes, probably 

 intruded shortly after the granitic intrusion. It has been stated 

 before that the age of this granite is unknown. If Archean, as has 

 been supposed, the slight amount of compression and change it has 

 undergone is certainly remarkable. A shearing is often noticeable, 

 dividing the rock into sheets or plates upward of a foot thick. The 

 direction of the shearing varies considerably, and is sometimes paral- 

 lel to the general direction of the quartz vein. In Gambrinus and 

 Sub-Rosa gulches a strike of -N. 20 to 24 W. and a dip of 70 E. or W. 

 were noted. In the Ranch Companj^'s claim at Placerville the sheet- 

 ing is parallel to the direction of the Quartzburg gold belt; strike, N. 

 45 E. ; dip, 60 SE. On Hawkins toll road, on the western slope of 

 the Boise Ridge, the strike is N. 60 to 70 W. and the dip 70 to 80 N. 

 or S. Conjugated systems of shear planes, having the same strike, 

 but dipping in opposite direction, thus occur here. In the depres- 

 sions and low ridges of the basin the granite is disintegrated to con- 

 siderable depth, so that it is very difficult to secure good specimens. 

 The disintegrated granite forms a coarse, yellowish-gray sand, the 

 individual grains of which have undergone but very little decompo- 

 sition, and which is easily swept down into the creeks by the rain 

 storms. Fresher and harder rocks crop out on the high ridge between 

 Elk and Moore creeks, upon which the Forest King and other mines 

 are located. The outcrops form brilliant white rounded masses ; but 

 even here the disintegration has made rapid progress. On the Boise 

 Ridge the granite is ordinarily soft and crumbling. The deepest 

 disintegration is probably found about the head of Muddy and Ophir 

 creeks, where a good outcrop is only rarely seen. The granite area 

 extends northward across the Payette River and far to the north of it. 



The granite has a coarse grain, the average size of the constituents 

 being 3 nim . The reddish orthoclase crystals are often very prominent. 



681 



