REVIEW AND GENERAL DISCUSSION. 519 
seem adequate to account for half the statements made, and I know not what other cause to 
assign. ‘- 
It is not uncommon to see the bedding of the gravel in the lower portions of the banks conform 
here and there, within certain limits, to the contour of the rock on which it rests, the strata curving 
gently upwards as they approach the rising rock. But I do not consider this as indicative of any 
disturbance since its deposition, except, perhaps, in certain cases on or near the rim-rock where 
land-slides may have affected it. In a little miner’s reservoir, a pertect section of which was after- 
wards exposed by the breaking of the bank through its centre, I have seen the lamine of mud 
and silt deposited in it, which were perfectly horizontal across its central portions, bend sharply 
upward — at angles as high, I think, as 15° or 20° — on approaching the banks at the edges of 
the reservoir. 
It is worthy, perhaps, of special notice, that what has been called, rather inaptly, I think, the 
“typical form” of the ancient gravel ridges, that is, the form which appears to be so well exem- 
plified in the Tuolumne County Table Mountain, and is illustrated in cross-section in Geology I. 
(page 248), is a thing of extremely rare occurrence. In fact, this Table Mountain is the only case 
1 know of in which a lava-stream has followed for any considerable distance the course of one of 
_ these ancient. channels, and covered it with a continuous protective capping of solid, compact rock, 
so that the axis of the ancient channel corresponds, after all the subsequent denudation, with the 
axis of the present ridge. And it is evident that, except in the case in which these two axes do 
essentially coincide, the typical, or at least the most frequent and characteristic forms of the ridges 
as exhibited in cross-section, cannot be that which is shown in the section above referred to. This 
form of a single great trough in the bed-rock, with rim-rocks rising high on either side, and running 
continuously parallel with the crest of the present ridge, is, indeed, I suppose, the typical or char- 
acteristic form for the cross-section of that Table Mountain ; but elsewhere it is rarely seen, and 
only for short distances, and probably as the result of accident in the relative courses of the ancient 
and modern streams. The width of the ridges, so generally and heavily capped with volcanic 
matter in the gravel region, varies from a hundred yards or less to several miles, and the surface of 
the bed-rock, as shown in the cross-section of one of these ridges, between the edges of the great 
cahons, would generally exhibit simply an irregularly undulating wavy line. This line would 
occasionally present tolerably distinct cross-sections of channels (generally of no great width or 
depth), and occasionally long, smooth stretches (where the line of cross-section might happen 
to strike and run for a little distance parallel with the bed of one of the earlier channels), while 
elsewhere it would be varied by knobs and irregularities of every kind, only with nothing 
sharply angular, but all smoothed and rounded by the action of running water. In fact, the 
ordinary form exhibited would be precisely that of the profile of a gently undulating, rocky 
country, over every foot of which the water has at one time or another held its course. And in 
making this statement I am not theorizing, but simply giving the fact, as shown by the vast ex- 
tent of drift and tunnel mining wherever I have been throughout the gravel region. Yet it must 
not be by any means supposed that all the bed-rock buried beneath the volcanic matter is so smooth 
as the preceding statement alone might seem to imply. Even in the broad basin of the Middle 
Fork of the American, where the above statement is eminently applicable, there were occasional 
bed-rock hills, rising several hundred feet above the beds of the earlier streams ; and sometimes 
these ancient hills rise now to the surface of the present volcanic capping, and so present themselves 
to-day as patches of bed-rock covered with only a partial coating of soil, and destitute alike of 
volcanic capping and of any considerable amount of gravel. 
A common and striking feature almost everywhere of the ancient gravel, and also to a great 
extent of the volcanic matter which overlies it, is the vast amount of chemical action which has 
taken place in the banks since they were deposited. The more or less complete decomposition and 
thorough softening of the hardest metamorphic and volcanic boulders, the formation at many 
localities of great quantities of semi-opal, which encrusts and fills the interstices between the 
pebbles, the frequent silicification of fossil-wood, its transformation at other localities into 
