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1 



REVIEW AND GENERAL DISCUSSION. 



523 



bits of quartz and metamorpliic 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 debris 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 volcanic ridge near Mokelumne Hill in Geology 

 1. (p. 207) 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 Avhich I could satisfy 

 mysdf 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 canons. 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 represents ! 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 iincorninon 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. I). W. 



