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REVIEW AND GENERAL DISCUSSION. 523 
bits of quartz and metamorphic slates which it contains enclosed, as proof positive against it. 
Some of them are smoothly water-worn, and some of them almost angular, but all clean and show- 
ing no effects of any high degrees of heat. 
Whether the fact that these fine materials form so largely the lowest beds of volcanic matter here 
to-day is any proof that they were the earliest ejected among the products of the volcanic action 
of the Sierra, I do not know. I can easily understand how the lightest materials may have been 
most quickly transported to considerable distances. And yet they must have been at least among 
the earlier products ; for it is extremely rare (if, indeed, it ever occurs) to find a bed of white 
lava overlying any of the coarser varieties of volcanic débris whose mass is so enormous in this 
country. 
There may possibly be another point of interest in this connection too. If the white lava and 
its allied materials are essentially rhyolitic in character, and the gray and darker-colored compact 
rocks which constitute the boulders and solid fragments in the conglomerates, the breccias and the 
volcanic gravels, are generally trachytic* in their nature, then this fact would seem inconsonant 
with Richthofen’s general theory ; for it would then appear, not as an isolated instance, but as a 
general fact, all over this section of the country, that trachyte had succeeded rhyolite in point of 
time of its ejection. I believe he applies his theory in its strictness, however, only to “ massive 
eruptions.” And if all the volcanic matter spread over this region be indeed the result of what he 
calls “volcanic action proper,” it might then be held, perhaps, that no anomalies which it may 
present can affect the general theory. 
The transition from these lower beds of ash and volcanic mud to the coarser overlying materials 
is not gradual but sudden. There are in general no intermediate steps or varieties. Immediately 
upon the white lava rests the volcanic gravel, the breccia, or the coarser conglomerate, as the case 
may be. Among the latter, indeed, there are beds of coarser and beds of comparatively fine ma- 
terial, and sometimes in different portions of the same bed variations may be noticed in the coarse- 
ness of the material. But these variations are rarely great, and I have never noticed them except 
in a few cases of comparatively thin beds. 
When I was in San Francisco in July and August, 1871, my attention was called to the descrip- 
tion of some curious facts in the structure of the voleanic ridge near Mokelumne Hill in Geology 
I. (p. 267), and after leaving town again in August, I watched constantly to see if I could find a 
parallel to it. I found plenty of instances which corresponded in part with that description, but 
not one which did in all. I found numerous cases where “the upper part of the ridge” was “a 
mass of boulders or fragments of trachytic lava, not polished nor smooth, but roughly rounded,” 
and this is the general character of the larger boulders in the coarser volcanic conglomerate 
throughout the country where I have been. But I never yet saw a case of a heavy mass of this 
material (and there are plenty of heavy masses of it almost everywhere) in which I could satisfy 
myself at all that the boulders were in reality “largest at the top” and grew “ quite small towards 
the bottom of the bed.” It is often true, indeed, especially where the crest of a ridge of this 
material is narrow, that the number of boulders visible, both large and small, is far greater on the 
crest and brows of the ridge than lower down its sides ; but it seemed to me that this was only 
a natural result of the slow action of the rains and melting snows, which might slowly remove the 
finer soil from even the crests of the ridges where it could not move the boulders, while lower 
down on the sloping sides of the ridge, the finer material could not be removed without at the 
same time undermining the boulders and leaving them free to roll down into the cafions. If this be 
correct, its direct result in time would be a great increase in the number of visible boulders on the 
crest and brows of the ridge, while the original proportion of boulders which the mass contained 
might be better represented lower down its sides. I also noticed the fact that when the ridges are 
* Microscopic examinations have shown that the dark-colored volcanic rocks to which reference is here made 
are chiefly andesitic in character. Trachytes are very uncommon in the Sierra. So far as relates to the order 
of succession enunciated by Richthofen, the bearing of the facts mentioned by Mr. Goodyear is not changed by 
the substitution of andesitic for trachytic. —J. D. W. 
