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 canons, 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 Eiver, 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 tin's material came here (the modern 

 canons 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 Placcrville. 



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. 





