308 RESUME AND THEORETICAL DISCUSSION. 
any way with those to the south, which have been already noticed ; but they 
have never received a careful examination at our hands. 
The region between the Middle and North forks of the Feather contains 
some very interesting deposits of gravel, which, however, are by no means 
of as much economical importance as those farther south. That they are 
not connected with the gravel channels in the Yuba Basin seems clear to 
Professor Pettee, as it did to the writer after a hasty reconnaissance in 1866. 
Indeed, we have within the area of a hundred square miles in the vicinity of 
Quincy, and including those remarkable deposits on the summits of Clermont 
and Spanish Peak, a number of localities of gravel at very different eleva- 
tions, and of very different lithological character, so that all attempt to co- 
ordinate them has thus far proved a failure. That there is in this region 
no system of ancient channels of Tertiary age comparable in magnitude 
with those farther south is evident, yet the geological problems presented by 
such occurrences as these on the two highest points of the region® are as 
interesting as those offering themselves in any portion of the Sierra. 
The foregoing rapid survey of the distribution of the gravels along the 
western slope of the Sierra naturally opens the way for the discussion of 
a number of important questions therewith connected. As, however, the 
volcanic formations are so intimately associated with those of aqueous origin 
that it is impossible to separate the two classes from each other in any ex- 
amination of the facts from the theoretical point of view, it will be well to 
give, before proceeding any further, a résumé of the most important condi- 
tions connected with the distribution of the masses of eruptive origin over- 
lying the bed-rock on the west slope of the Sierra Nevada. After that, we 
may proceed to consider some of the more striking peculiarities of the grav- 
els, as regards their lithological character and general mode of occurrence. 
A large amount of detail has been given in the preceding pages, and in 
the Appendix, with regard to the character and position of the volcanic for- 
mations associated with the gravels in the Sierra Nevada. What remains to 
be done in this section is to present this branch of the subject in a more 
general way than has previously been possible, although want of space makes 
it necessary to do this in the briefest possible manner. 
In the first place, as to the general fact that the gravels are associated 
with volcanic deposits. No one can doubt this for a moment who only 
inspects the various maps and diagrams accompanying the present volume. 
* See ante, pp. 215, 216, 
