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REVIEW AND GENERAL DISCUSSION. 



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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 volcanic instead of metamorphic materials. 



The coarser volcanic 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 



treams, 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 debris, 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 

 lono-, 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 1 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 oc- 

 curred 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 debris 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 bo 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 fir up toward the summit, yet 

 it is also a general rule that above a certain altitude the ancient gravel is less smoothly washed, 

 and o-rows more and more angular the higher we go. And both the facts last stated appear to me 



