1916J 



Dickerson: Tejon Eocene of California 



411 



volcano of Marysville Buttes, clays, sands and gravels of Miocene age have 

 been brought up by the intrusion of igneous bodies and, although they are 

 so much disturbed that the stratigraphic sequence can not be made out, 

 there is strong probability that these strata should be identified with the 

 lone. They contain marine fossils associated with impressions of deciduous 

 leaves, and the gravels contain some gold. 



South of Lincoln the lone formation is better exposed because it has 

 been protected by andesitic tuff, but it does not reach a higher elevation 

 than about 200 feet. South of American River the outcrops are more exten- 

 sive, and the formation attains its greatest development in the foothills of 

 Calaveras County. The lower part, consisting of white clay and sand, reaches 

 a thickness of 860 feet or more and contains beds of lignite of poor quality. 

 Above this rests a white sandstone which attains a thickness of 100 feet or 

 more. A clay bed also of light color, 100 feet thick, overlies this sand- 

 stone. Near Valley Springs the lone attains elevations of 1000 feet, and 

 its highest members are probably several hundred feet above the deepest 

 gravels of the Tertiary Calaveras River, which here debouches into the plains 

 but which is not visible in this vicinity. 



From all this it appears that during the later part of the prevolcanic 

 gravel period the lone formation transgressed along the whole front of the 

 Sierra to a present elevation of somewhat more than 1000 feet. So far as 

 can be judged from the present exposures, there has been little differential 

 elevation along the front of the range since the time of the deposition of 

 the formation. In other words, the fluctuations of the shore line are now 

 indicated by horizontal lines at least between the Oroville Table Mountain 

 and the foothills of Calaveras County. 



The extent of the erosion which followed immediately after the deposition 

 of the lone formation along the foothills was greater than would be sup- 

 posed from a study of the deposits in the rivers higher up in the range. 

 The sequence is particularly well shown around the Oroville Table Mountain 

 and in the foothills of Calaveras County. At the former place the andesitic 

 tuffs (Tuscan tuff of Diller), which in Placer County appear to overlie the 

 lone formation conformably, are at least 500 feet below its top members. 

 At the mouth of the old Yuba River there is a conspicuous absence of 

 the lone formation and the andesitic tuffs rest immediately on the bedrock, 

 at elevations as low as 200 feet, and in the old river channel lie immediately 

 above the heavy gravels which are presumably of Eocene age. At this 

 locality no extensive mud flows of rhyolitic character appear to have reached 

 the valley, although they are abundantly present in the longitudinal basin 

 which begins at North Columbia, Nevada County, 15 miles east. At the 

 mouth of the Tertiary Calaveras River, on the other hand, shore gravels 

 or delta gravels spread out up to a present level of 500 feet, and these 

 gravels rest on the gently eroded surface of the lone formation. 



These interrhyolitic gravels were again subjected to some erosion; imme- 

 diately after this followed the prolonged epoch of andesitic flows. The tuffs 

 which were spread over a large part of the Sierra Nevada were worked 

 over by the rivers and spread as thick masses of volcanic gravels and sands 

 over the eroded surface of the lone formation and the interrhyolitic gravels. 



The ' ' gravels of interrhyolitic age ' ' resting upon ' ' the eroded sur- 

 face of lone", described by Lindgren are in reality of andesitic or 



