

GEOLOGY OF THE SIEERA NEVADA. 



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formations to be described in the next chapter. These are the marine 

 sedimentary strata found in the foot-hills at numerous points all along 

 the eastern margin of the Great Valley. These marine deposits, like those 

 higher up in the mountains, rest unconformably on the upturned edges of 

 the auriferous slates. They are usually well supplied with fossils, so that 

 there can be no doubt as to their geological position, or the nature of the 

 medium in which they were deposited. They include strata both of Tertiary 

 and Cretaceous age ; and, although there arc localities where they might be 

 confounded with the true auriferous gravel deposits of fresh-water origin, 

 there is usually no difficulty in distinguishing the two formations, as will be 

 evident from the following brief sketch of the character and mode of occur- 



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rence of the marine strata. 



The greatest development of these deposits is in the southern part of the 

 Great Valley, and especially along the foot-hills between Kern and White 

 rivers. Here this formation is from 200 to 600 feet in thickness, and it is 

 either horizontal or has a slight dip to the east, away from the mountains, 

 from the erosion of which its material appears to have been derived. The 

 rock is usually a soft sandstone, chiefly made up of granitic debris. From 

 Kern to King's River the older metamorphic rocks come down to the edge 

 of the valley^and are there covered by recent detrital beds. Farther north, 

 and extending to the Stanislaus, there is a belt of low flat-topped hills of 

 sandstone, very much eroded, so as to leave many little isolated patches, of 

 from a few feet up to 150 in elevation. All these deposits to the south 

 of the American River appear to be of Tertiary age, and a portion of them 

 were identified by Mr. Gabb as Miocene. The upper portion of some of these 

 Tertiary hills arc of fresh-water origin, containing bones of land animals and 



d. There are also localities where the volcanic detrital 

 material has found its way entirely down to the level of the Great Valley, - 

 an occurrence which will be easily understood, after the description of the 

 auriferous gravels and associated volcanic materials in the next chapter has 



been consulted. 



On the American River, just below the town of Folsom, the Cretaceous 



formation makes its appearance, and there are other small patches of the 



same, well filled with fossils, in the vicinity. Farther north the Cretaceous 



occupies a considerable area, the best exposures being found to the north of 





fragments of woo 



Oroville. This formation is cut throu 



trh in the foot-hills by the streams 



coming down the slope of the Sierra, and there are excellent exposures of it 



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