THE FOSSILS OF THE GRAVEL SERIES. 



219 



CHAPTER III. 



THE FOSSILS OF THE AURIFEROUS GRAVEL SERIES 



I 



i 



Section* I. — Introductory Remarks. 



Nothing has been said in the preceding pages of the geological age of the 

 series of deposits included under the comprehensive term of " gravels.'' As 

 will have been seen from the detailed description of the mode of occurrence 

 of these accumulations of detrital material, there is a mass of sedimentary 



i 



material often one or two hundred feet in thickness, and sometimes three or 

 four times that, piled up on the upturned edges of the bed-rock series, and 

 one of the important questions to be answered in regard to this formation is : 



What is its geological age ? To answer 



this we must turn to the investiga- 



tion of the fossils which it contains, and which — as has already been hinted 



several times in the preceding pages — are quite numerous and varied in 

 character. 



The fossiliferous contents of the strata belonging to the gravel series group 

 themselves naturally in three divisions : 1. The microscopic organisms ; 2. The 

 plants ; 3. The animal remains. Each of these will be considered separately 

 in a special section of this chapter. 



The division between the microscopic organisms and the plants proper is, 

 of course, more or less an arbitrary one. Many of the bodies found in 

 these strata, and too small to be seen by the naked eye, are in fact commi- 

 nuted fragments of undoubted vegetable forms, such as the carices and grasses 

 proper ; while other, and in many localities almost the whole of them, belong 

 to more obscure forms, — so obscure, indeed, that there is not an entire 

 unanimity of opinion among naturalists as to whether they are really to be 

 classed in the animal or in the vegetable kingdom. 



While the infusorial deposits of the gravel series, or those containing mi- 

 croscopic organisms, are both abundant and important in the Sierra Nevada, 

 they are of little value as aiding in the determination of the geological age 

 of the formation. The remains of plants, on the other hand, which are 

 found abundantly in the formation in question, are of great value in that 





