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I 



REVIEW AND GENERAL DISCUSSION. 



519 



seem adequate to account for half the statements made, and I know not what other cause to 



assign. 



This 



v 



It is not uncommon to see the bedding of the gravel in the lower portions of the banks conform 

 hero 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 perfect section of winch was after- 

 wards exposed by the breaking of the bank through its centre, I have seen the laminm 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 

 tlio rcsorvoir 



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 exeni- 

 plilied in the Tuolumne County Table Mountain, and is illustrated in cross-section m 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! 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. T 

 form of a single great trough in the bed-rock, with rim-rocks rising high on either side, and run mm 

 continuously parallel with the crest of the present ridge, is, indeed, 1 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 tin. ..ravel 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 

 canons would generally exhibit simply an irregularly undulating wavy line. This hue 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 bo 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 m 

 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, whore 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 fdls the interstices between the 

 pebbles the "frequent silicification of fossil-wood, its transformation at other localities into 





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4XHMHH 



