Vol. 6] 



Beid: The Geomorphogeny of the Sierra Nevada. 



99 



but none on the east side. Gabbro at present exists only south- 

 west of Tahoe City. The metamorphic andesites correspond with 

 those of the Mt. Tallae area and the area west of Carson. The 

 porphyritie boulders are derived possibly from the diabase- 

 porphyrite area to the west. From what little is known of the 

 whole of the gravels, it can be stated that they have closest re- 

 lations with the rocks of the west and southwest. This conclu- 

 sion, if true, means that the river flowed eastward from a divide 

 some distance west of Lake Tahoe. Possible evidence of east- 

 ward flow is also found in the gorge-like portion of the old 

 channel on the fault-block just east of Lake Tahoe, now filled 

 with rhyolite capping the gravels. East of Washoe Lake, in the 

 Virginia Range, scattered boulders have been found in a few 

 places, and a bed of gravel over an acre in extent is found north 

 of Carson at the point where a w T et-weather stream debouches 

 upon Eagle Valley. All these probably represent transposed 

 remnants of the old river channel. Nothing definite can be 

 stated for the area further east. 



Volcanies. — The volcanic rocks are of various types and ages. 

 The relative ages are for most of the species yet in doubt, for 

 but few contacts are visible. A glance at the map will make the 

 reason for this clear. In the Sierra Nevada proper the volcanies 

 exist only in isolated patches. The two important members of 

 this group are rhyolite and andesite. Both are found capping 

 the river gravels, yet nowhere come into contact. The texture 

 of the various residual spots of andesite is not everywhere the 

 same, and sufficient variation exists to justify the belief that 

 there is some difference in age. This, however, is yet an open 

 question. Northeast of Glenbrook there are dike-like occurrences 

 of andesite within the large area of the same rock. South of 

 Glenbrook, just beyond the edge of the andesite flow, occurs a 

 circular neck of andesitic rock rising vertically out of the granite 

 that is probably of earlier age than the flow. West of Lake 

 Tahoe Lindgren describes the rhyolite below the andesite. In 

 the Truckee Canon east of Reno, as characteristic of the Vir- 

 ginia Range, the andesite is on top of the rhyolite. Hence for 

 the present the greater age of the rhyolite near Carson will be 

 assumed. 



