■ a ^j^w' I »* - 





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344 



RESUME AND THEORETICAL DISCUSSION. 



The fact that the large bodies of detrital 











namely, their elevated position, 

 material described in the preceding pages occupy the summits of the country 

 or the divides between the present rivers in the hydraulic mining region 

 seems at first view to be contrary to what ought to be expected ; yet a care- 

 ful consideration of the facts, in connection with what has been laid clown in 

 the preceding pages with reference to the nature of the erosion effected by 

 gradually diminishing streams of water, will, as the writer believes, enable us 

 to o-et over some of the difficulties which this branch of the subject presents. 

 There are, however, points which seem involved in great obscurity; and even 

 in regard to some of those for which an explanation is here offered there 

 will, no doubt, be lack of harmony of opinion among geologists. 



The reader may obtain an idea of the difference in relative height between 

 the gravels and the present river valley by examining the sections on Plate 

 G. Figure 1, to which reference has already been made/* illustrates by the 

 relative grades of the channels in the most important portion of the hydraulic 

 mining region, namely, that in the American and Yuba Kivcr basins. In 

 preparing this diagram, the elevations, as determined by the Survey, of numer- 

 ous points, both at the level of the present streams and on the bed-rock in 

 the ancient channels, were laid down and then connected by lines, the broken 

 ones designating the existing rivers, and the full ones those of Pliocene age. 

 With regard to the South Yuba, there can be no difficulty in recognizing its 

 former representative ; in the region drained by the numerous branches 

 of the American River, there was more room for fancy, in connecting the 

 various localities so as to reproduce continuous channels which should be 

 undoubted representatives of the present streams. For all purposes, however, 

 of seeing at a glance the general difference of level between the Pliocene 



and the present rivers, the diagram is quite as valuable as if there could be 

 no question, in any case, in regard to the proper manner of connecting the 

 different gravel localities, so as to exhibit the relationship between ancient 

 and modern rivers with exact truth. 



Figure 3, Plate G, to which reference has been made in Appendix A,t 

 shows the difference of level along a line drawn in a southeasterly direction, 

 — that is, nearly parallel with the range of the Sierra, — from near La Porte, 

 for a distance of about twelve miles, across four deep canons, and indicates 

 the position of the high gravels with reference to the depressions occupied 

 by the present streams. 





* See ante, p. 64. 



t See Appendix A, Section VII. 



