524 PECULIARITIES OF THE VOLCANIC DEPOSITS. 
broad (and they are sometimes several miles in width) though there are occasional patches where 
the boulders are very thick, especially about the slopes of the little gulches, etc., yet the general 
rule is that the boulders visible on the flat surface of the ridge are not so very numerous ; but 
almost without exception, on approaching the caiions, I found them very numerous about the brow 
of the hill, while again they were generally not so numerous lower down its side. Moreover, 
wherever the boulders at the top were large, I hardly ever failed on hunting to find occasional 
large ones also sticking low down in the sides of the ridge. Furthermore, I have seen a good 
many sections of this kind of material from thirty to fifty feet in height, and sometimes consider- 
ably more, exposed by hydraulic mining, and I never yet saw such a section in which the 
boulders appeared, as a rule, any larger near the top than they did lower down. I think, therefore, 
that the structure of this ridge at Mokelumne Hill (which I did not visit) must be very peculiar 
in this respect. 
But there is another statement respecting that locality to which I have not been able to find a 
parallel anywhere else. It is this: ‘There are no other kinds of rock than volcanic represented 
in this bed, and no stray pebbles even of quartz or slate ; this fact is not peculiar to the lava ridges 
of Mokelumne Hill, but has been observed in many other places in this region.” I cannot help 
thinking the fact stated in the last clause quoted above a very extraordinary one, for I have 
never yet found a locality on any of the volcanic ridges over which I travelled last year where 
there were “no stray pebbles, even of quartz or slate.” I have, indeed, seen places enough where 
they were scarce; in fact, they are generally so, and oftentimes extremely rare ; but often when, 
in riding along for some little distance, I could see none of them, I have dismounted and looked 
for them, and in such cases, rare as they might be, I have never failed to find them after a little 
search. There are plenty of them, that is, comparatively speaking, on the ridge between Sutter 
Creek and Jackson. And if there is any resemblance in the general physical character of the vol- 
canic matter in the ridges on the opposite sides of the Mokelumne River, I can hardly think that 
their entire absence is a common thing in the region southeast of that stream. In fact, I should 
really be more disposed to question the thoroughness of any ordinary search which might have been 
made for them, than I should be to believe in their entire absence. 
Nor have I seen any place in this part of the country where it appeared to me necessary, or even 
admissible, to suppose that a ‘mass of lava had been broken up while flowing and before it was 
entirely consolidated,” the fragments being “ pushed over each other by the pressure of the mass 
from behind,” and thus becoming rounded by their own friction alone. I acknowledge that there 
are difficulties, which I cannot remove, in the way of fully and satisfactorily explaining just how 
so many volcanic boulders, of such huge dimensions as some of them are, have been transported 
so far, and distributed without sorting at every height above the bed-rock from top to bottom, 
through such masses of finer fragmentary material ; but considering the stray pebbles of quartz 
and metamorphic rocks and granite, which, so far as my own observation has extended, the mass 
of this material always does contain, I know of no means which seems to me capable of having 
accomplished the work without the aid and instrumentality of water in some way. And when 
we consider the vast period of time which has elapsed since this material came here (the modern 
caiions have all been excavated since then), in connection with the slow effects of weathering, etc., 
on even the hardest rocks, I think it will hardly do to argue against the agency of water in some 
way in the matter, simply because the boulders are “not polished or smooth” to-day, and appear 
but roughly rounded. 
From the description given* of the material underlying this mass of boulders at Mokelumne 
Hill, it evidently belongs to the same class of material that I have constantly called “ white lava,” 
after the local name by which it is known at Placerville. 
With reference to the manner in which the breccias were distributed over the country, I am 
much inclined to think that they travelled in the form of mud-flows. I know not how else to 
account for the sharp angularity of their rocky fragments, together with the presence in them of 
* See Geology, Vol. I. p. 268. 
