REVIEW AND GENERAL DISCUSSION. 513 
are nevertheless perfectly well marked, and it is evident almost everywhere that the auriferous 
gravel had been slowly gathering for a very long period of time before the first volcanic matter 
came. The occurrence of the white lava boulders in the lowest gravel, as at some localities in 
the vicinity of Placerville, is a rare and abnormal thing; and therefore, though this gravel be the 
lowest at these places, and rests immediately upon the bed-rock, I am nevertheless disposed to 
consider it of somewhat more recent origin than the great mass of the gravel which underlies the 
volcanic matter. The general fact is that, although metamorphic gravel may here and there be 
found interstratified with the volcanic matter at all heights from the bottom of the latter up 
to the present surface, yet there is a horizon below which nothing volcanic can be found in the 
banks, and everything between this horizon and the surface of the bed-rock is metamorphic 
gravel.* 
As already repeatedly stated, every particle of volcanic matter in the region through which I 
travelled in 1871 is fragmentary. There are no lava-streams, and not a pound of all the vol- 
canic matter that I saw between the Mokelumne River and the North Fork of the American 
(unless it be the Jackson Butte) appears to rest to-day where it was when it last cooled from 
igneous fusion. I think the whole of it has been transported and distributed in one way or 
another through the agency of water; and I consider it certain that, at least during the earlier 
portion of the volcanic era, a very large proportion of it was brought down the mountains and 
spread over the country by means precisely similar to those which had previously distributed the 
gravel, that is, by shallow and shifting streams of running water; and the immense quantities of 
smoothly washed and perfectly rounded volcanic gravel which exist near Placerville, and also at 
sundry localities farther south and southeast, prove also that the same agency was still active at a 
far later period in the history. But there are also vast accumulations of volcanic breccias whose 
material has evidently not been subjected for any considerable length of time to the action of 
running water. And there are also enormous quantities of voleanic matter which is neither a 
breccia nor properly speaking a gravel, but which I have often called conglomerate, and which is 
known all through the southern part of Placer County as simply “ cement,” or sometimes “‘ moun- 
tain cement.” It is filled with hard boulders of volcanic rock of all sizes up to many tons in 
weight, these pebbles and boulders being generally but partially rounded ; and the finer material 
which constitutes their matrix is dark colored, and has apparently been produced by the trituration 
and pulverization and decomposition of rocks of similar kinds to the boulders which it encloses. 
This material can hardly have been distributed, I think, by the ordinary action of ordinary streams, 
but there is no lack of agencies by which both its distribution and that of the breccias may have 
been brought about. The volcanic energy which once displayed itself on so grand a scale in 
the High Sierra can hardly have been unattended by a class of phenomena so common in active 
volcanic regions now. The sudden melting of great masses of snow ; the occasional ejection from 
beneath the surface of bodies of water, either hot or cold; the sudden deluges of rain which are 
among the meteorological effects of eruptions ; the quick, overwhelming flow or the slow, majestic 
creep of great volumes of volcanic mud, etc.,—all these, and more, are probably among the 
agencies which have contributed from time to time to the distribution of these materials. The 
scattering of ashes in showers through the air may also of course have aided in the work, although 
the probability is that little if any of the material so distributed remains at present where 
it fell, having been quickly washed off the hillsides by the rains and redistributed by the 
streams. 
Thus it is probable that the slow and ordinary working of the streams was from time to time 
interrupted by grander paroxysmal action, during which great quantities of material were rapidly 
spread over extensive areas, while during the intervals between these paroxysms the streams again 
resumed their action and their general course, though rarely perhaps following exactly the same 
” 
* Wherever I use the words ‘‘metamorphic gravel” without further explanation, I mean by them not 
gravel which has been metamorphosed since its deposition, but simply gravel which has been derived from the 
metamorphic rocks. 
