50 G. P. BECKER — STRUCTURE OP THE SIERRA NEVADA. 



Page. 



Origin of the Forces 70 



Thesis of a solid Earth maintained 70 



Direction of Subsidence 70 



Experiments on Subsidence 71 



Direction of Upheaval 72 



Geological Application 72 



Application to the Sierra 73 



Summary of Observations. 



Region studied. — During the past field season I liave made studies of the 

 structure of a part of the high Sierra with a view to elucidating the orogenic 

 forces involved. My observations were made chiefly from the south fork of 

 the Stanislaus river to the Truckee, a distance of about 80 miles, and cov- 

 ered a belt of some 30 miles in width immediately west of the eastern scarp 

 of the range. The results are in some respects incomplete even for this 

 limited area, but they appear to be of sufficient interest to justify a pre- 

 liminary paper. During the field-work I did not find myself able to frame 

 any hypothesis to account for the phenomena, although these were in some 

 of their features singularly regular. This was in one respect advantageous, 

 for it left me free from all bias in the collection of facts. On the other hand, 

 it led to a failure to record minutely certain features of structure which I 

 now believe capable of very complete explanation. 



Mocks and Exposures. — The rocks throughout the area mentioned are 

 chiefly granite and diorite, overlain in part by andesite and basalt. Except- 

 ing portions of the eastern slope of the range and the higher peaks, the wjiole 

 area has been glaciated up to the summits of the passes and, as the glaciation 

 is very recent, the rock is admirably exposed. 



Fissures. — The granite and other granular rocks are intersected by fissures 

 at short intervals. Sometimes these appear to be without any regularity ; 

 but much more often they are manifestly grouped in systems. A few square 

 miles will be particularly characterized by horizontal fissures and here the 

 granite mountains will appear terraced. In such cases vertical or diagonal 

 fissures are always present, but are less prominent than the horizontal part- 

 ings. In other areas the granite will be intersected by diagonal fissures 

 usually dipping at about 45° and striking between northwest and north. 

 Tliese dip in both directions and divide the mass into horizontal columns. 

 Much more frequent than any other fissure systems are vertical partings, and 

 these are remarkably uniform in strike, almost always running either nearly 

 north-northwest or at right angles to this direction. As a rule, where these 

 vertical systems occur together, one of them is more strongly pronounced 



