4 



University of California Publications. 



[Gfologt 



on the map PI. 1 in the northeast corner. It also exposes 

 several of the formations. 



The second and third ridges are separated by a broad valley 

 known as Mason Valley, of about ten miles width, and through 

 it the river finds its course. Two streams, the East and West 

 Walker, unite about two miles south of the country mapped in 

 PL 1 in this same valley, and form the main river. 



DISTRIBUTION OF FORMATIONS. 



The First Ridge. — The "Benchland" in the northwest corner 

 of the map is made up for the most part of andesite tuff and 

 breccia, and is capped by a nearly level lying later andesite. The 

 thickness of the cap is not uniformly the same, but the sections 

 that were closely inspected were less than twenty feet. The 

 andesite breccia in the tuff underlying the cap is entirely dif- 

 ferent from the cap, hornblende being a striking constituent. 

 The tuff possesses occasionally some stratification. The erosion 

 of the canon has not cut entirely through the tuff, and an esti- 

 mate of its thickness was not made. A few occasional springs 

 make their appearance along a "wash" which is in the bottom 

 of the canon, and these are presumably where bedrock comes 

 near the surface. Near one of these a knob of granite may be 

 seen. At the northern end of this first ridge there are a few 

 small and irregular areas of shale and limestone. These occur 

 in a mass which appears to be one of the earliest intmsives. It 

 is a porphyrite which will be described later. Rhyolite presents 

 itself throughout the remainder of the ridge, with the hornblende 

 andesite protruding through it in a few places, and also one 

 small knob of granite near the southern end. The granite, shale, 

 limestone and porphyrite is a basement complex corresponding 

 to the Bedrock Series of the Sierra Nevada. The rhyolite, ande- 

 site and tuff may he regarded as members of a series that in a 

 similar manner correspond to the Superjacent Series of that 

 region. The remaining territory promises for the two major 

 divisions, the members already mentioned and others additional. 

 A couple of small exposures of iron-ore are at the base of the 

 ridge ; one near the southern end and the other with one of the 

 limestone areas near the northern end. 



