Vol. 4] Smith. — Upper Region of Main Walker River. 



9 



rhyolite are here well revealed, as may be seen by consulting 

 Fig. 1. The basalt rests on the sandstone that overlies the rhyo- 

 lite. 



The Mid-valley Buttes. — The broad valley below the second 

 or Sing-ats'-e Ridge is studded with four buttes which peer up 

 through the alluvium. The northern one, Mason Butte, is com- 

 posed of granite and dykes of hornblende andesite. There are 

 no less than seven or eight of these that cut through the butte 

 lengthwise. Another butte southeast of this one is made up of 

 granite and rhyolite, and across the river are two more buttes, 

 of which the more westerly is granite and granite-porphyry 

 similar to that already mentioned in Sing-ats'-e Ridge, and the 

 fourth butte is granite and earlier intrusives similar to that 

 mentioned in the first ridge. 



UNCONFORMITIES. 



The sedimentaries occupy about one-thirtieth of the entire 

 area. There is enough of them and a few fossils, however, to 

 give information as to their age and the structure of the region. 

 The range of the sedimentaries is from the end of the Palaeozoic 

 or early Mesozoic to the present, as the fossils show. The Creta- 

 ceous and the Eocene are not recorded by any accumulations, but 

 there are indications of considerable erosion which took place 

 within a time that may correspond to these two periods. There 

 was no section found where the entire column could be measured. 



The Bedrock Complex and the Tertiary. — The unconformity 

 between the shale and limestone of the Bedrock Complex, and 

 the beds of the Tertiary is usually very marked. The amount of 

 distui'bance and deformation of the complex is considerable, 

 while that of the Tertiary is not so great. A few fossils indi- 

 cate the limestone to be Triassic. In the Tertiary one pebble was 

 found containing a fossil, which was submitted to Dr. J. P. 

 Smith of Stanford University for determination of stratigraph- 

 ical position, and the following communication was received : 

 "The black silicious rock contains what appeal's to be Daonella, 

 a pelecypod characteristic of the Middle Trias. The Geological 

 Survey of California discovered some Triassic fossils 1 at Vol- 



1 Described in Amer. Jour, of Coneliology, Vol. V, by W. M. Gabb, 

 ' ' Description of some Mesozoic Fossils, ' ' etc. 



