Vol. 6] Beid: The Geomorphogeny of the Sierra Nevada. 103 



region can be established. If this be done, a great step in ad- 

 vance toward unraveling the complete geological history of the 

 Sierra Nevada will have been taken. 



The earliest lava now about which we have definite evidence 

 is the rhyolite. This capped the old river gravels before a large 

 amount of the faulting had occurred. The lowest parts of the 

 rhyolite are breccias. Above and next in age is the pitchstone, 

 in the few small patches noted. Andesite, perhaps as an earlier 

 and a later flow, is next in the series, although some andesite 

 may antedate the rhyolite. Basalt represents the last volcanic 

 activity in the region. Of the absolute age of these lavas we as 

 yet know little. Even the relation of the basalt to the Carson 

 lake beds is not known by direct evidence. The lake beds are en- 

 tirely granitic in the valley below the basalt, and the now is 

 probably later and therefore of late Quaternary age. 



Influence op Eocks Upon the Geomorphy. 



Before proceeding to a consideration of the geomorphy it 

 becomes necessary to ascertain the influence of the various tec- 

 tonic units upon the land form. The main elements of the geo- 

 morphy are due to causes that have no direct ascertainable re- 

 lation to the rocks of the Superjacent Series. In the Sierra 

 Nevada these later rocks have at the most a slightly modifying 

 effect upon the form. The chief forces that have acted upon the 

 range are those of the atmosphere and those of diastrophism. 

 The volcanics are removed by erosion so rapidly compared with 

 the granodiorite and schist that they have left no great effects 

 upon the topography due to their original forms. Diastrophic 

 movements have been most severe since the volcanic period and 

 have faulted all rocks alike. Hence we are chiefly concerned 

 with the Bedrock Complex in the present connection. 



Joints. — The granodiorite and schist are intricately jointed. 

 The joints are persistent through both rocks irrespective of their 

 physical differences (see pi. 21b) and are always along planes. 

 Their position bears no relation to the surface, either present or 

 ancient. This statement is based both on observation and on 

 the nature of the physiographic form. Upon the old erosional 

 surface and the more recent fault-scarps the faults could have 



