52 G. F. BECKER STRUCTURE OF THE SIERRA NEVADA. 



Miocene. Then trouble began again. The early Tertiaries of the Coast 

 ranges were thrown into folds, the axes of which are nearly parallel to the 

 coast, and the Miocene of the Great Basin was similarly affected. The main 

 volcanic eruptions of the Sierra probably began at this time, or perhaps a 

 little earlier. They continued throughout the Pliocene. 



Since the Assuring and faulting of the granite amounts to a very great 

 disturbance, which one can hardly suppose simply local, it is natural to refer 

 them to the Pliocene. They were also connected with the andesitic erup- 

 tions. The dikes of andesite which are found near superficial masses of this 

 lava, and which sometimes connect distant areas of andesite, follow the 

 vertical fissure systems under discussion. This has been noted by Professor 

 Reyer* and I have abundantly confirmed his observations in this respect. 

 Even if the fissures were substantially older than the andesite, the intrusion 

 of this rock must have caused renewed movements of the granite, and as a 

 matter of fact the faults of greatest throw which I have found immediately 

 adjoin intrusions of andesite along the fissures. A portion of the faulting 

 is thus certainly referable to the period of andesitic eruptions, and I have 

 as yet found no indication that the fissure systems under discussion antedate 

 this period. 



Abseiice of post- Glacial Faults. — It is well established that there have been 

 recent faults in the Great Basin as far west as the eastern base of the Sierra 

 Nevada, and post-glacial faults have also been reported from the higher 

 portion of the range.t The area of glaciated surfaces' is enormous, and I 

 confidently expected to find many post-glacial faults, even if they had a 

 throw of only a fraction of an inch. In spite of daily searches for many 

 weeks, I was unable to make certain of any such movements. There are, 

 indeed, cases in which dislocation of glaciated surfaces has occurred, but all 

 the instances of this kind which have come under my observation appear 

 distinctly superficial and due in all probability to the action of the frost, 

 more or less assisted by alternations in temperature. I met with no cases 

 that simulated post-glacial faults near the bottoms of canons or where the 

 rock was manifestly solid. At the very beginning of the investigation I 

 became aware that in dealing with faults of small throw it is needful to be 

 constantly on the watch for the dislocations of a merely superficial character 

 which are so common in mountain ranges, and I have found proof of such 

 movements in which surprisingly large masses of granite were involved. It 

 is j^ossible that I may have examined post-glacial faults without recognizing 

 them, for much of the granite decomposes readily, and some comparatively 

 recent faulted edges may have become rounded by decomposition ; but if 

 there has been post-glacial faulting in the area under discussion it has been 

 of small amount. 



* E. Reyer's " Theoretische Geologie," 1888, p. 537. f Ibid., p. 638. 



