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514 



CHARACTER OF THE LAVA-FLOWS. 





channels as before. The working of the streams themselves was doubtless as varied and complex 

 as the work of such shifting streams, on such a slope and in such circumstances, could be; and ex- 

 cavation along some portions of their channels was probably almost as constant a fact as accumu- 

 lation was at others, though the localities of both were constantly shifting. Yet the prevailing 

 tendency was still to accumulation, until the era of great eruptions had passed away, and the vol- 

 canic period was drawing toward its close. Then, and not till then, did the excavation of the 



great modern canons in reality begin. 



It is a common idea that, because the volcanic matter upon the western slope of the Sierra is 

 now found almost exclusively upon the crests of the present ridges, the capacity of this volcanic 

 capping to resist erosive and denuding action is greater than that of the hard metamorphic slates 

 themselves, and therefore that this capping, wherever it occurs, has acted as an effectual protection 

 to the bed-rock beneath it against the erosive action of the modern streams. Almost a necessary 

 corollary to this idea was the inference that the areas now occupied by the modern canons had 

 never been covered by any considerable quantity of volcanic matter and some have gone so far 

 as to believe that the volcanic matter only flowed along the beds of ancient canons, between 

 which, and exactly over the modern canons, then existed great and high-projecting bed-rock ridges, 

 so that since the volcanic era the whole topographical relief of ridge and canon on the western 

 slope has been completely reversed, — exactly inverted. 



E"ow I believe that nothing could be further from representing the real facts and the actual his- 

 tory of the case, as a general rule, than the above suppositions are. There are instances, indeed, — 

 like the Table Mountain of Tuolumne County, — where actual lava-streams have solidified into 

 hard and compact basalt, and wherever this has occurred the basalt is of course far harder than the 

 slates, and has afforded an effective protection to them. But even in this case, though it is self- 

 evident that the molten lava must have followed in its flow the lowest channel it could find, it 

 does not follow, from this alone, that the adjacent country need have been much, if any, higher 

 than what was required to simply confine the stream within its banks. At all events, there is no 

 necessity for assuming on this account, without other facts to prove it, that above the plane of the 

 surface of the lava-stream the adjacent country towered in high mountain masses where now the 

 whole surface of the region is hundreds of feet below the level of the flow. 



But again, I am satisfied that such lava-streams, which have flowed for any considerable distance 

 down the southwestern slope of the central and southeastern Sierra, are few and far between. 

 Indeed, from Plumas County southeast to Mariposa, or even to Walker's Pass, throughout the 

 whole region over which the ancient auriferous gravel is distributed on the southwestern slope be- 

 tween these limits, a region in places from thirty to fifty miles in width, I do not believe that the 

 total volume of the lava-streams constitutes even so large a proportion as one per cent of the vast 

 ao-oregate of volcanic matter which is spread over the country. It is not likely that there is a lava- 

 flow anywhere below the altitude of 6,000 feet on the southwestern slope, between the Central 

 Pacific Eailroad and the Mokelumne River. At least, I did not see one that I could recognize as 

 such. And of all the varieties which help to make up the enormous aggregate of fragmentary vol- 

 canic matter spread over that region, there is not one, so for as I have ever seen, that would offer a 

 greater resistance to erosive action than the bed-rock itself. Of all these varieties, so far as my 

 observation extends, the hardest are some of the breccias. Yet even the hardest of these would 

 be eroded by running water with greater ease and greater rapidity than even the upturned edges 

 of the greater portion of the hard, tough bed-rock would ; while vastly the greater portion of all 

 the volcanic matter here is far softer, and would be eroded with much greater ease. I believe, 

 therefore, that, instead of the volcanic matter's protecting the bed-rock beneath it, the very ease 

 and rapidity with which the volcanic matter was first eroded has, in many parts of the country at 

 least, contributed in no small degree to increase the subsequent rapidity of erosion of the bed- 

 rock itself in the excavation of the tremendous modern canons. The basin of the Middle Fork 

 of the American River, east and southeast from Michigan Bluff, is a good illustration of this. 



Few streams in the Sierra have deeper canons than the various forks of the American River. 



