REVIEW AND GENERAL DISCUSSION. 51] 
doubt or question, first, that the blue lead theory is not only utterly inadequate to account for 
the facts, but is directly in conflict with them; second, that there has neither been any great 
upheaval nor any considerable disturbance of any kind in the bed-rocks of the Sierra since the 
gravel period ; third, that the general course and direction of the drainage of the range was the 
same in the gravel period that it is to-day; and fourth, as shown by evidence already adduced, 
that the situation and general outlines of the larger drainage basins themselves were also then 
essentially the same as now, while the instances are not rare in which the resemblance may be 
traced even somewhat further yet into detail. 
But though the general drainage system was thus the same, yet many of the most important 
circumstances attending its action were vastly different. 
Before going further, I may simply mention one other theory, occasionally ventilated in the past, 
respecting the ancient gravel, and remarkable only for the peculiar facility with which its inventor 
compels grand rivers to follow the dictates of his fancy. This theory supposes that during the 
gravel period the Columbia River, by some mysterious and occult means, found its way to the 
western slope of the central California Sierra, and then flowed southeast for an indefinite distance, 
thus furnishing the requisite water, and constituting the great blue lead. I merely notice two 
objections to this theory: the first one is that the blue lead never existed ; and the second 
is, that, even if it had existed, I know of no definite correlative facts which could furnish any 
better reason for identifying it particularly with the Columbia River than with the Colorado, the 
Mississippi, or the Ganges. 
I propose now to consider in a few words the general surface and aspect of the western slope at 
the commencement of the gravel period, and then to sketch as briefly and definitely as possible the 
general outlines of what I conceive to be the history both of the gravel and the volcanic periods in 
the regiqn of country where I travelled in 1871. After this, there are certain special questions 
which I will consider somewhat more in detail. 
At the commencement, then, of the gravel period there were no great cafions in the gentle 
southwestern slope of the Sierra, nor in all probability any volcanic matter spread over the exten- 
sive region through which the ancient gravel is now distributed. If any volcanic matter then 
existed here at all, its quantity must have been extremely small, and all the evidence known to me 
points to the conclusion that there was none. Whatever may have been going on at that time 
along the summit or in the country to the east of the Sierra, or whatever may have been the 
exact date of the first volcanic outbreaks there, it was certainly not until long ages afterward, and 
near the close of the gravel period, that the volcanic energy of the Sierra reached its grander devel- 
opment, or that the materials ejected began to find their way to any considerable distance down the 
' western slope. Indeed, I am much inclined to think that the earliest voleanic phenomena of the 
Sierra did not occur until very late in the gravel period; for there are many facts which appear 
to me indicative of a strong probability that, after the first outbreaks, the development of the 
voleanic energy was rapid as well as great towards the maximum of its grandeur, though we know 
that its after subsidence was slow and long. But more of this hereafter. 
The southwestern slope of the Sierra Nevada, then, at the commencement of the gravel period 
was a broad, gently undulating, and moderately hilly country, having in all probability exactly 
the same gentle average slope which exists to-day southwesterly towards the valley, —a slope 
which for great distances ranges from one hundred to one hundred and thirty feet to the mile, — 
and having at the same time its greater drainage basins, already marked out where they exist to- 
day, by the occasional ridges and spurs which traversed, sometimes for considerable distances, 
the rolling hilly country, though rarely rising so much asa thousand feet above the general 
plane. High up towards the summit of the range, indeed, the country grew gradually rougher ; 
and here there was probably more or less of cahon and of precipice, though it is not at all 
likely that even here there was anything comparable in magnitude with many of the moderna 
cafions. 
There may possibly, indeed, have been among the peaks occasional gorges two or three thousand 
