334 RESUME AND THEORETICAL DISCUSSION. 
in making the necessary preparations for successful work. Many of the 
prominent points connected with this branch of the inquiry have already 
been up for consideration in the preceding pages; and some of the difficulties 
which present themselves have been discussed both by Professor Pettee and 
Mr. Goodyear in their contributions to this volume. The general mode of 
occurrence of the gravels may be considered as having been well made out: 
what remains to be done is to inquire more particularly into the causes by 
which these results have been brought about; and here we enter a field 
beset with difficulties, one in which but little work has yet been done, and 
where there seems to be but little harmony of opinion among the few careful 
observers who have entered it. The writer will, however, present, necessarily 
somewhat briefly, the theoretical results to which he has been led by a some- 
what protracted study of the region in question. 
We seem now to have arrived at the proper point for instituting a closer 
comparison than has hitherto been made between the high gravels and the 
detrital accumulations which we see forming at the present day ; or, in other 
words, between Tertiary and Recent deposits. As already mentioned in a 
general way, quantity and elevation are the main characteristic features of 
the older gravel masses. 
In the first place, we must notice the character of the present river chan- 
nels, and the cations of which they form the bottoms. These caions, through- 
out the gold region, are very much of one type; they are deep,* their walls 
sloping steeply and being almost entirely free from débris. The amount of 
detritus in the beds of the streams is very small as compared with the size 
of the gorge or excavated V-shaped depressions, the lowest points of which 
they occupy, with rarely any considerable breadth of comparatively level 
ground on either side. Thus,a stream of only a few feet in width may often 
be seen at the bottom of a gorge the walls of which rise directly from the 
water on both sides, at an angle of 30° or 40°, and to a vertical height of 
two thousand feet and more. 
The quantity of water carried by the different streams descending the 
slope of the Sierra, in the gold region, varies exceedingly from season to 
season, as well as from year to year. In the case of those which do not head 
in the very highest portion of the range, the amount becomes towards the 
close of the summer reduced almost to nothing. Even a river draining so 
large an area as the Yuba is but little more than a rivulet in August and 
* See ante, pp. 64, 65. 
