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PECULIARITIES OF THE GKAVEL. 



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sulphuret of iron, the irregular cementing together of the originally loose materials, and the forma- 

 tion in many places of the large quantities of sulphuret of iron which often contributes so 

 largely both to the hardness and to the peculiar color of the " blue gravel," are all examples of 

 its effects. So also is the commonest condition of the bed-rock immediately underlying the gravel, 

 which, far oftener than otherwise, is more or less thoroughly softened and decomposed. 



I do not think this chemical action has been in general either the effect of thermal springs, 

 or of anything else which has issued from the bed-rock beneath. Nor do I see the necessity of 

 supplying any other agency to accomplish it than filtering atmospheric waters. The atmospheric 

 waters, in filtering through such masses of volcanic debris as cap these ridges, would undoubtedly 

 slowly attack them and derive from them certain substances which they would carry forward in 

 solution, and which might be peculiarly adapted to act upon the metamorphic material beneath. 

 When we consider also the presence of the organic matter in the banks, and the very complex 

 chemical constitution of the banks themselves as a whole, I think that, with the atmospheric waters 

 and the lapse of ages, we have all that is necessary to account for a vast amount of chemical action. 

 Even the metamorphism (for it is really such, I think) of some of the fine and white volcanic 

 ashes, included in the popular name " white lava," into a compact, almost clinkstone-like material, 

 enclosing small crystals of glassy feldspar, may probably have been effected by similar means. 

 And, in connection with this point, it is interesting to note the fact that at certain localities, as, 

 for instance, in the bluff near the toll-house above Smith's Flat on the Placerville and Carson 

 road, at the Jackson Valley Euttes, etc., the sharply granular white lava has become incrusted 

 on its weathered surface by a crust from an eighth of an inch to half an inch thick, of perfectly 

 similar compact material, which sometimes has almost a semi-vitreous lustre and appearance, and 

 which can have been produced in loco only by a change in the texture of the material. 



In this connection it may be well to notice an impression which is not uncommon among the 

 miners, that " the gravel is always of the same color as the bed-rock on which it rests," that is, 

 that if the bed-rock immediately underlying the gravel be hard, undecom posed, and dark-bluish in 

 color, then the gravel immediately upon it will be " blue gravel " ; but if the bed-rock be decom- 

 posed and white, or of any light color, stained reddish or yellowish, then the gravel upon it will 

 be red or yellow. Now it is true, indeed, that this is very often the case, and I have no doubt 

 that in many instances both the color of the bed-rock and that of the gravel upon it have been due 

 to the same cause, namely, the character and extent of the local chemical action in the banks. But 

 it is by no means always so, and there is certainly no lack of instances, as at Yankee Jim's, where 

 red or yellow gravel rests immediately on dark-blue bed-rock. 



A curious question relating to the western slope of the Sierra is the origin of its diamonds. 

 They have been found in circumstances which prove that they came from the ancient gravel, and 

 therefore originally, in all probability, from the bed-rock itself. But from what particular variety 

 of rock, and under what circumstances, were they formed ? Also, why is it that none have yet been 

 found (so far as I know, at least) farther northwest than White Rock, near Placerville, while by far 

 the greatest masses of the ancient auriferous gravel are northwest of the North Fork of the 

 American ? 



In may be noted that not infrequently, at different localities not far removed from each other, 

 the gravel is so entirely different in both its lithological and physical characteristics that this fact 

 would be sufficient of itself alone to prove a difference of local origin. This is eminently the case 

 at Muletown and Irishtown, northwest of lone Valley. 



I do not know how to account for such large accumulations as exist at some localities of nearly 

 pure quartz gravel, except upon the supposition that this gravel has not travelled very far, and that 

 the enclosing rocks from which it came were soft enough to disintegrate and be carried oil' by 

 the water as sand and mud. 



It is an interesting fact that, so far as I was able to learn, there are along the edge of the Sacra- 

 mento Valley, skirting the foot-hills between the American and the Mokelumne rivers, no large 

 accumulations of gravel at all, except in the immediate vicinity of the points where the present 





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