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THE AURIFEROUS GRAVELS OF THE SIERRA NEVADA. 



as well as chemical change, since its deposition. In general the strata are 

 very considerably elevated, and often nearly into a vertical position. This 

 portion of the mass of the Sierra is universally known to the miners by the 

 comprehensive term of "bed-rock/' a term the origin of which is easily un- 

 derstood; it is the foundation on which repose the more recent superficial 

 accumulations which form the mantle spoken of above. These latter are 

 easily distinguished by their unconformability of position, their non-crystal- 

 line character, and the fact that they are usually not compacted into what 

 the miner would recognize as a rock, or a material which cannot be excavated 

 without blasting. It is true that in other mountain chains the older forma- 

 tions often have resting upon them in places a layer of detrital material, in 

 the form of gravels, sands, clays, and soil ; and in a range like the Scandi- 

 navian, which has been during a long period covered with snow and ice, or 

 glaciated, as it is called, such accumulations often acquire a very considerable 

 thickness. Indeed, the superficial detrital covering of the "rock in place," 

 as geologists term it, is always of importance, if only from the fact that the 

 agricultural character of the country is so largely dependent on its nature 

 and distribution. In the case of the Sierra Nevada, however, there are 

 several reasons why the formations in question — those which overlie the 

 bed-rock ■ — are of greater interest than they are in perhaps any other 

 mountain chain. They are often of great thickness, as compared with the 

 usual development of such detrital beds ; they represent in the epoch of their 

 formation a period of geological time of very considerable length, not being 

 exclusively of recent or post-tertiary origin ; they are quite largely made up 

 of volcanic materials, so that it is easy to see that the period during which 

 they have been accumulating has been one of great igneous activity; and, 

 finally, they are, almost everywhere, more or less auriferous, containing a suf- 

 ficient quantity of gold to make their working profitable. It is in reality 

 this last-named quality which makes these superficial deposits of so much im- 

 portance. If they contained no gold, we should also know much less about 

 their mode of occurrence ; for it is the twenty-five years' work of thousands 

 of miners among these auriferous deposits which has revealed their real 

 nature, of which hardly anything could have been made out from a study of 

 the undisturbed surface. In giving a brief sketch of the geology of the 

 Sierra Nevada, before entering on the more detailed study of the super- 

 ficial detrital and volcanic formations, we begin with the bed-rock; and 

 first, with some hints as to its lithological character. 



