30 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 
uature, 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. 
