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REVIEW AND GENERAL DISCUSSION. 525 
fossil leaves, so delicately preserved as those at Negro Hill near Placerville. There are, indeed, 
some difficulties connected with this supposition, one of which is the difficulty of understanding 
how mud of such a character, that is, so full of angular, rocky fragments, could have flowed so far 
over such gentle grades, and in such thin sheets as we sometimes find among these breccias. Yet 
this is decidedly the most plausible idea that occurs to me, all others being fraught with greater 
difficulties still. 
The volcanic gravel (known at Placerville under the name of “ mountain gravel,” and at other 
localities as “black gravel,” etc.) is of course the work of running streams, precisely like the 
metamorphic auriferous gravel, only consisting of voleanic instead of metamorphic materials. 
The coarser voleanic conglomerates (I call them such for want of a better name, though oftentimes 
they are not consolidated into a rocky whole, but only form rather compact banks), with their roughly 
rounded and sometimes enormous boulders, still remain. Their materials are not sorted, so far as 
I have seen, in any way, with respect to coarseness or fineness, and I am by no means certain as to 
the means by which they were transported. They can hardly be the work, I think, of constantly 
running streams, of any magnitude whatever; for it seems hardly possible that any such streams, 
of sufficient force to have brought these enormous boulders over such gentle grades, should not 
have carried all the fine material farther on, instead of leaving it here to form the matrix in which 
the boulders are imbedded. Nor does it seem likely that they were simple mud-flows ; for if the 
mud were fluid enough to flow so far over such gentle slopes, it appears probable that such heavy 
boulders would have forced their way downward through the mass, and sunk to the bottom. 
May they not, perhaps, have been brought here by a series of occasional sudden and heavy 
floods, bringing down with them vast quantities of débris, and spreading it quickly over consider- 
able areas at a time, each flood of this kind being succeeded by a period of quiet, no matter how 
long, if only long enough to allow the surplus water to ooze and drain out from the newest arrived 
material, and the mass to become comparatively dry and somewhat compacted together? In this 
way the largest boulders of the latest flood might be left on top of even the finest material of the 
preceding one, and thus, as the mass gradually accumulated, these boulders might become imbedded 
in it at every height above the bed-rock from the bottom to the top. It is true that, if this be 
the case, I do not know what caused the floods. But it is not difficult to point out agencies 
which might produce them in a high volcanic mountain country. The only positive evidence that 
I know of against this idea (if indeed it be such) is the rarity of any distinct traces of definite 
bedding or stratification in heavy masses of this material. But of all the ideas which have oe- 
eurred to me in connection with this difficult problem, this supposition appears to me to be liable 
to the least objections, and in reality the most plausible ; especially when we consider the fact 
that in this peculiarity of distribution of the heaviest boulders throughout the banks, from the 
bottom to the top of the mass, we may perhaps find an exact parallel to this material in the “ sage- 
brush slopes” of granitic débris in Owen’s Valley, where, at all events in many places, huge 
boulders rest upon the very surface of the slopes at points far distant from the foot of the moun- 
tains, and where I feel confident that some such agency as this has aided in their distribution. 
The question, how far up the mountains towards the summit the ancient gravel extends, is one 
to which a definite answer can hardly perhaps be given. Mr. Sterrett, near Canada Hill, told me 
of one acquaintance of his who had found well-washed metamorphic gravel beneath the volcanic 
matter at the very summit; and I consider this by no means an impossibility, though perhaps 
hardly probable ; but it certainly extends very far up, and may be found here and there beneath 
the volcanic matter to within short distances of the summit, if not actually to it. It is the general 
fact, however, that after we rise above a certain level, the aggregate quantity of the ancient gravel 
diminishes, and then grows less the higher we go; and, though there is hardly a rule of any 
kind relating to the ancient gravel which has not here and there its apparent exceptions, and 
though we may occasionally find very smoothly washed gravel very far up toward the summit, yet 
it is also a general rule that above a certain altitude the ancient gravel is less smoothly washed, 
and grows more and more angular the higher we go. And both the facts last stated appear to me 
