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76 



THE AURIFEROUS GRAVELS OF THE SIERRA NEVADA. 



Such remaining portions are known to the miner as the " rims/' or the "rirn- 



' To one who knew nothing of the peculiarities of the gravel mining 



rock. 



the 









region, it would seem very strange to see — as may often be done — 

 miner running a tunnel in the hard bed-rock, where there are no external indi- 

 cations of gravel deposits whatever, and especially when it is ascertained that 

 the work on such a preparatory undertaking, which can never begin to pay 

 until gravel has been reached, may require years for its completion and cost 

 many thousands of dollars. The miner, in such a case, if questioned as to 

 the object of his work, would say that he was " tunnelling through the rim- 

 rock, in search of the channel." His knowledge of the country would have 

 taught him that there was a sufficient chance of finding a paying deposit of 

 gravel, occupying a channel-like depression somewhere in the region towards 

 which he was tunnelling, to make it worth while for him to risk his time and 

 money in the manner suggested. Some of these channels in the Sierra are 

 wonderfully well defined and deep, with a perfectly preserved rim on each 

 side, as will be seen from the descriptions farther on. 



In many places there is associated with the gravel, often overlying it, but 

 sometimes in interstratified beds, a large amount of material of a volcanic ori- 

 gin. This may be cither solid lava, in the form of a regular flow consolidated 

 from a liquid condition, or it may be of a detrital character, and made up of 

 more or less rounded fragments, which after having been ejected have been 

 carried to a distance from the place of their origin by currents of water, and 

 deposited as a stratified mass of volcanic conglomerate or breccia. Some of 

 these deposits seem to consist of material which has fallen into the water, in the 

 form of ashes and lapilli, and taken on a decidedly stratified form, somewhat 

 resembling clay in appearance. Not unfrequently volcanic and ordinary 

 sedimentary strata alternate with each other, or occur mixed together in the 

 same layer, so that it is not easy to decide, without close examination, of 

 what the deposit really consists. For the different kinds of volcanic material 

 occurring with the gravels there are various names in use among the miners. 

 In the regions where the fragmental character predominates, as is often the 

 case, especially in the central mining counties, the volcanic deposits are called 

 « cement." This is a term somewhat differently employed in different dis- 

 tricts, but most frequently applied to rather closely compacted volcanic 

 breccias and conglomerates, of which the different coarser materials seem 

 to be pretty firmly cemented together by the, finer portion of a very similar 

 material. The whiter, fine-grained and homogeneous beds resulting from 











