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CHAPTER IV. 



BfiSUMfi AND THEORETICAL DISCUSSION OF THE PRINCIPAL FACTS CONNECTED 



WITH THE GEOLOGICAL OCCURRENCE OF THE AURIFEROUS GRAVELS. 





Section I. — Introductory remarks. 



In a previous chapter of this work a brief general account of the mode of 

 occurrence of the Auriferous Gravels of the Sierra Nevada has been already 

 given, for the purpose of making that which was to follow intelligible to the 

 reader. It now remains to present a fuller discussion' of a portion at least of 

 the problems involved in the gravel question, in which the object will be, to 

 throw some light on the physical conditions which have had as a result the 

 very remarkable deposits of which a somewhat detailed description has been 



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given in the pages of this volume. 



It will be well to caution the reader, however, that an exhaustive discus- 

 sion of the phenomena of the gravels is not to be expected. Indeed, such is 

 the nature of geological inquiries in general, that in no branch of that science 

 is there a final and absolute result to be attained. The geologist must content 

 himself with slow and often hardly perceptible advances towards the ideal 

 of a perfect work ; he must be satisfied if he throws here and there a little 

 light on doubtful points, removes a few erroneous ideas, and indicates to 

 some extent the direction wljich future investigations should take. Further- 

 more, it should be mentioned in this connection that the present volume is 

 not intended to answer as a technical guide to the practical side of hydraulic 

 mining. The processes employed in that kind of work are simple enough in 

 theory, and the machinery not complicated ; notwithstanding this, the busi- 

 ness is one which requires great experience and much caution, since the 

 preliminary expenditures in such undertakings must of necessity be large, 

 and difficulties are wont to occur for which only the most far-sighted are 

 likely to be prepared. 



An ideal report on the gravel region, from the practical point of view, 

 would demand a much larger amount of time and a vastly greater expendi- 

 ture of money than have been at the disposal of the present writer. Such a 

 report is not to be looked for, indeed, if by an " ideal report " we mean one 



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