REVIEW 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 cafions 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 them overflow their banks ; 
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 cafions 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 caiions 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, ete. 
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 caiions. This is the work which has deepened the cafions, while their correspond- 
ing widening has been chiefly due 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 caiions with 
enormous masses of débris, 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 cafions. I now propose to consider further some special points of interest in connection 
with the general 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 gravel nor voleanic matter in the hills southwest of it, while both are found to the north- 
