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REVIEW AND GENERAL DISCUSSION. 



500 



being still supposed to be nearly parallel, and the general direction of their flow to be south- 

 easterly. 



Such, if I understand it correctly, is the general outline of the blue lead theory. It was neces- 

 sary to state it, in order to present more clearly and definitely the objections against it. So great 

 was the confidence in this theory that Richthofen has drawn important inferences from it * in 

 relation to great disturbances of the Sierra Nevada, and more than one prominent gentleman 

 has believed that he had actually traced the blue lead from Sierra County to Placerville, if not 



beyond. 



The first objection to this blue lead theory lies in the fact that from the Middle Yuba to the 

 Mokelumno River, including the best known and most important gravel mining region in the 

 State, there is no well-defined belt or belts of deep gravel banks stretching northwesterly and 

 southeasterly, such as must necessarily underlie the very foundation of this theory. 



In one case, indeed, there is for a few miles — that is, from Gold Run to the Forest Hill ridge 

 — a well-defined belt or line of deep banks, extending, not southeast, but nearly south. Rut 

 beyond the points named in this direction it cannot be traced. Going south from the Forest Hill 

 rid<»\> there are no more heavy banks of ancient auriferous gravel until we reach Placerville ; ami 

 at Placerville the character of the banks is in some important respects different from anything we 

 find between Gold Run and the Forest Hill ridge. 



Moreover, in going southeast from Placerville we find no more heavy banks until we get at 

 least beyond the Mokelumne River. It must be noted that iVi this connection I am speaking not 

 of thin sheets of gravel, or of smaller channels in which the gravel may be from one to six or eight 

 feet in thickness, or even sometimes eighteen or twenty feet, but of great accumulations, from 

 seventy-five to one hundred and fifty or two hundred feet in thickness, of mctamorphic gravel, such 

 as may be supposed to mark the courses of the greater of the ancient streams. 



A^ain in going north or northwest from Gold Run, the "belt" in question can be no longer 

 traced, for another reason, namely, it becomes lost in the broad extent of country over which the 

 heavy banks are spread from Smartsville, almost at the foot of the mountains, to Dutch Flat, 3,400 

 feet above the sea and occurring at too many points within this range of altitude, as at San Juan, 

 French Corral etc., to justify any inference of a general southeasterly flow of the streams which 

 fathered it. I speak still of heavy banks and not of lighter deposits and smaller channels. But 



the latter are scattered for and wide over the western slope of the Sierra beneath the volcanic 

 matter covering a vast aggregate area, through a range of more than 6,000 feet of altitude and 

 over a breadth of from thirty to fifty miles of country, and distributed in such a way as can hardly 

 with possibility, and certainly not with any approach to probability, be accounted for on the blue 



lead theory. 



The single fact of the non-existence of any such definite northwest and southeast belt or belts 

 of deep travel banks as have been supposed, though perhaps insufficient of itself alone to positively 

 disprove the blue lead theory, does nevertheless completely remove the very foundation on which 

 it was built. But there are other and more positive objections to it. Before speaking of them, 

 however, I will state incidentally another important fact connected with this theory. The term 

 ''blue lead" derived its origin, in part at least, from the blue gravel, and there has always been 



an intimate association in ideas between the two, many even going so far as to believe that the 



blue gravel was characteristic everywhere of the blue lead, and was not found outside of it. Now 

 the fact is that the blue gravel is scattered here and there all over the gravel region, though its 

 quantity is ordinarily the greatest where the banks are heaviest. It cannot be said to be charac- 

 teristic of any particular channel or set of channels. And I am thoroughly satisfied that the 

 peculiar dark-bluish color which gave rise to the name "blue gravel " is due, in the vast majority of 



cases at least not to any original peculiarity in the character of the rocks from which the gravel 

 was derived nor to any peculiarity in special streams which once flowed in particular channels, 

 but simply to local chemical action in the mass of the gravel itself subsequent to its deposition. 



* See The Natural System of Volcanie Rocks, pp. 8ti and 87. 





















