298 RESUME AND THEORETICAL DISCUSSION. 
it will not be desirable to take the space which would be necessary for stat- 
ine them, in advance of their presentation in the proper order as developed 
in the ensuing pages. 
Section Ill. — The General Distribution of the Detrital and Volcane Materials 
on the West Slope of the Sierra Nevada. 
In proceeding to a general discussion of the phenomena of the gravel 
deposits, it will be desirable, in the first place, to give an account of their 
geographical distribution, and of the various materials of which they are 
made up. 
In considering the position of the hydraulic mines of California, one could 
hardly fail to be struck at once with the fact that they are concentrated 
within quite a limited space as compared with the whole area of the State. 
It will also be apparent that gravels which are auriferous in character occur 
where no hydraulic mining has been attempted; and, furthermore, that large 
deposits of superficial detritus exist where no mining at all is carried on. 
Each of these conditions has its special and rather complicated set of causes, 
the nature of which has already been explained, to some extent, in the pre- 
ceding pages,* but in regard to which a few words must be added in the 
present connection. 
Supposing that gravel existed in large quantities in any region, the first 
question in regard to working it would be, Is it auriferous? If this were 
found to be the case, the next point would be, the quantity of water which 
could be obtained and the cost of the same. The existence of hydraulic 
mines in any region implies the presence of large bodies of auriferous gravel, 
and also of facilities for obtaining large quantities of water at a cost not too 
high in proportion to the yield of the gravel. There may be so much gold 
present that a high price can be paid for water, or a heavy expenditure in- 
curred in bringing it to the spot; or, on the other hand, there may be so 
little gold in the gravel as to make the obtaining of it a remunerative enter- 
prise only when water is abundant and cheap. Again, the deposits of detrital 
material may be fairly rich, or even very rich, and yet too thin to make it 
worth while to establish the necessary plant for hydraulic mining, which 
necessarily presupposes the occurrence of the material on a large scale. 
Another set of conditions connect themselves with the nature of the operations 
by which the gold-bearing detritus is handled and with the profit resulting 
* See ante, pp. 74-78. 
