76 THE AURIFEROUS GRAVELS OF THE SIERRA NEVADA. 
Such remaining portions are known to the miner as the “rims,” or the “ rim- 
rock.” To one who knew nothing of the peculiarities of the gravel mining 
region, it would seem very strange to see —as may often be done — the 
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 either 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 
