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11EVIEW AND GENERAL DISCUSSION. 



515 



In this portion of the basin of the Middle Fork, also, there can be little doubt that the volcanic 

 matter once covered the areas of the present canons as well as the tops of the modern ridges. All 

 this region at the close of the volcanic period appears to have been a smooth and gently sloping 

 plain, consisting mainly of volcanic debris to depths of from 300 or 400 to 600 or 800 feet, under- 

 laid by the auriferous gravel resting on the rock. When now the streams came, after this, to 

 follow once more their quiet courses across the surface of this loose material, they would naturally 

 excavate little channels in it ; and the supplies of new material from higher in the mountains being 

 partially at least shut off by the cessation of the eruptions, the tendency of every freshet afterwards 

 would be to deepen more and more the channels already begun. From this time forward the 

 excavation would proceed more and more rapidly till a depth of channel was reached such that the 

 greatest floods could no longer raise the streams sufficiently to make thorn overflow their banks : 



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and, finally, when the streams in their gradually sinking beds had reached the surface of the solid 

 rock, they would be already confined in the bottom of narrow canons from 300 to 800 feet 

 deep, with sides as steep as the material would lie, and would have reached their maximum of 



excavating power. 



When the volcanic period passed away, and the streams were first left free to choose anew their 

 courses over the newest surface of the ground, the slightest inequalities in the form of that surface 

 would of course suffice to influence or determine the local courses of the streams. These courses 

 would therefore be chosen and determined without any reference whatever to the network of older 

 channels, now so deeply buried under them, beyond the single fact that they would drain nearly the 

 same regions of country, and in the same general direction. The newer channels would therefore 

 cross the courses of the older ones at innumerable points and in every possible relative direction, 

 cutting them up into detached pieces and patches of network, whose sections, when afterwards 

 exposed, would appear as complex and intricate as the work and the history of the earlier streams 



had been. 



The courses of the new streams over the smooth surface might have been at first easily changed, 



and it seems not at all unlikely that, if man had been here then, it might have been even within 

 his power to have guided their courses, and thus remodelled the whole present system of canons in 

 this portion of the basin of the Middle Fork. But when the streams, in a later period of their his- 

 tory, reached the surface of the solid rock beneath, then they had already gathered a power which 

 in flood-time nothing could resist, and which even projecting knobs and spurs of most compact and 

 solid rock could hardly turn aside from its direct and onward course. From that time forward the 

 excavation of the solid rock itself went on, as it does now, varied only, so far as we know, by varia- 

 tions in the quantity of water, due to climatic changes attendant upon the glacial epoch, etc. 

 When the streams were low, they possessed, like the present ones, little or no excavating power ; 

 but whenever the floods came, and. they were swollen to great and roaring torrents, carrying not 

 simply sand and pebbles, but even great boulders of tons in weight, thundering along their beds, 

 then the wear was great and the excavation rapid. 



The actual direct work of the streams themselves has always been confined, of course, to the 

 bottoms of their canons. This is the work which lias deepened the canons, while their correspond- 

 ing widening has been chiefly duo to another cause, namely, the land-slips and slides, which from 

 time to time rushed down from the too steep sides, often filling the bottoms of the canons with 

 enormous masses of debris, which, however, were quickly swept away. 



Such is a sketch of what I conceive to be the general outline of the history of the gravel and 

 volcanic periods in this portion of the country, together with the subsequent excavation of the 

 modern canons. I now propose to consider further some special points of interest in connection 



with the gen ciid subject. 



The axis of the Calaveras County Bear Mountain range lies northwesterly and southeasterly, 

 and I know of no volcanic matter or ancient gravel among the hills to the southwest of this 

 ridge. The length of this ridge is some twelve or fifteen miles ; and if it be true that there is 

 neither travel nor volcanic matter in the hills southwest of it, while both are found to the north- 



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