ran 



n«w, 



m* r*i •■s 



■ 



REVIEW AND GENERAL DISCUSSION. 



511 



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 Eiver, 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 Eiver 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 region 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 canons 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 volcanic 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 

 volcanic 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, — aslope 

 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 as a 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 canon and of precipice, though it is not at all 

 likely that even here there was anything comparable in magnitude with many of the modern 

 canons. 



There may possibly, indeed, have been among the peaks occasional gorges two or three thousand 









