GEOLOGY OF THE SIERRA NEVADA. 29 
those included, for convenience, under the general term of “ auriferous 
gravels.” 
In studying the Sierra Nevada, we have first to make ourselves familiar 
with the fact that, in so far as its geological structure is concerned, we have 
to do with two quite distinct series of formations. The principal mass of this 
chain is made up, as is usually the case with great ranges of mountains, of 
hard, crystalline rocks; a portion of these is evidently of intrusive origin,— that 
is, it has come up from beneath and forced its way through the other por- 
tion. The material which has played this part of an elevating nucleus or axis 
is granite. The rocks through which the granite has made its way are chiefly 
of sedimentary origin; but they have undergone a large amount of chemical 
change, by which they have often been rendered crystalline, and their origi- 
nal character has been so modified, that in some places it can no longer be 
recognized. Thus far the Sierra Nevada resembles many other mountain 
chains; but in the case of most of the great ranges we find the formations 
growing less crystalline as we recede in either direction from the central 
axis. We observe, also, that these less altered groups of strata are of more 
recent geological age, and it is usual to find in mountain chains of the first 
magnitude a considerable part of the geological series represented. The 
change from one formation to another is, however, usually a gradual one, 
and the whole mass is found to have partaken more or less in the processes 
of disturbance and uplifts by which the chain has been formed. Thus we 
have in the Alps a range remarkable for the number and variety of the flex- 
ures of the sedimentary strata, which are lapped over the crystalline nucleus; 
but these flexures extend up to and include rocks of Miocene Tertiary, as 
well as of Liassic, Jurassic, and Cretaceous. The same is the case with the 
Himalaya, and many other chains. There is, of course, in such cases, no easily 
recognized and marked line of division in the series of formations of which 
the range is made up. In the Sierra Nevada, on the other hand, there is a 
most decided separation of the materials forming the mass of the mountains 
into two groups. There is the granitic axis with the associated crystalline! 
rocks, and an overlying mantle, if the term may be allowed, of rocks quite 
different in lithological character and in position, the two series being so dis- 
tinct from each other, in every way, that they could by no possibility be con- 
founded even by the most unobservant mind. The lower or interior portion 
is usually hard and crystalline, and if its planes of stratification can be made 
out, it is easily seen that the rock has undergone more or less disturbance, 
