376 RESUME AND THEORETICAL DISCUSSION. 
auriferous. deposit is already almost entirely worked out; in others, the top 
gravel has been washed away, leaving the lower beds, which may be much 
richer than the upper ones, and, at the same time, more difficult of attack. 
Again, there are regions where a vast amount of gravel remains untouched, 
on account of the heavy capping of volcanic materials, either too hard to be 
hydraulicked, or too thick to be washed away with profit. In other districts 
there is still much gravel of low tenor to be washed, whenever the capital is 
forthcoming to supply water at a sufficiently low price. Again, there are 
apparently magnificent fields for enterprise, ready to hand, in laying open 
as yet almost untouched deposits, where the chief obstacle to development 
has hitherto been the difficulty of consolidating claims, by purchase or other- 
wise, without very large expenditures. 
On the whole, it is evident from a consideration of the facts laid before the 
public in the present volume, that the hydraulic mining business may still 
be successfully prosecuted through many years before the field is exhausted. 
And it is probable that for quite a number of years to come the product of. 
gold will not be materially diminished. The most serious obstacle likely to be 
met with is the filling of the valleys with tailings, thus hindering washing, and 
also exciting alarm and opposition on the part of the agricultural population 
of the belt of land adjacent to the foot-hills. Here complications may arise, 
putting a less favorable aspect on the hydraulic mining interest. Indeed, 
the possible condition of the western slope of the Sierra after the streams 
have become choked with tailings, the gold mines exhausted, and the prin- 
cipal part of the timber cut down, is not one satisfactory to contemplate. 
There is no part of the world where scientific oversight and judicious legis- 
lative interference is more desirable for the future welfare of the community 
than in the Sierra Nevada of California, 
