334 



RESUME AND THEORETICAL DISCUSSION. 







in making the necessary preparations for successful work. Many of the 

 prominent points connected with this branch of the inquiry have already 

 been up for consideration in the preceding pages ; and some of the difficulties 

 which present themselves have been discussed both by Professor Pettee and 

 Mr. Goodyear in their contributions to this volume. The general mode of 

 occurrence of the gravels may be considered as having been well made out: 

 what remains to be done is to inquire more particularly into the causes by 

 which these results have been brought about; and here w6 enter a field 

 beset with difficulties, one in which but little work has yet been done, and 

 where there seems to be but little harmony of opinion among the few careful 

 observers who have entered it. The writer will, however, present, necessarily 

 somewhat briefly, the theoretical results to which he has been led by a some- 

 what protracted study of the region in question. 



We seem now to have arrived at the proper point for instituting a closer 

 comparison than has hitherto been made between the high gravels and the 

 detrital accumulations which we see forming at the present day ; or, in other 

 words, between Tertiary and Eecent deposits. As already mentioned in a 

 general way, quantity and elevation are the main characteristic features of 

 the older gravel masses. 



In the first place, we must notice the character of the present river chan- 

 nels, and the canons of which they form the bottoms. These canons, through- 

 out the gold region, are very much of one type ; they are deep,* their walls 

 sloping steeply and being almost entirely free from debris. The amount of 

 detritus in the beds of the streams is very small as compared with the size 

 of the gorge or excavated V-shaped depressions, the lowest points of which 

 they occupy, with rarely any considerable breadth of comparatively level 

 ground on either side. Thus, a stream of only a few feet in width may often 

 be seen at the bottom of a gorge the walls of which rise directly from the 

 water on both sides, at an angle of 30° or 40°, and to a vertical height of 

 two thousand feet and more. 



The quantity of water carried by the dine rent streams descending the 

 slope of the Sierra, in the gold region, varies exceedingly from season to 

 season, as well as from year to year. In the case of those which do not head 

 in the very highest portion of the range, the amount becomes towards the 

 close of the summer reduced almost to nothing. Even a river draining so 

 large an area as the Yuba is but little more than a rivulet in August and 



* See ante, pp. 64, 65. 





