REVIEW AND GENERAL DISCUSSION. 509 
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 nore 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 
Mokelumne 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. But 
beyond the points named in this direction it cannot be traced. Going south from the Forest Hill 
ridge, there are no more heavy banks of ancient auriferous gravel until we reach Placerville ; and 
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 in 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 metamorphic gravel, such 
as may be supposed to mark the courses of the greater of the ancient streams. 
Again, 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 
gathered it. I speak still of heavy banks and not of lighter deposits and smaller channels. But 
the latter are scattered far 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 gravel 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 Voleanic Rocks, pp. 86 and 87. 
