344 RESUME AND THEORETICAL DISCUSSION. 
namely, their elevated position. The fact that the large bodies of detrital 
inaterial 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 down 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 get 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 River 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 im 
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 caiions, and indicates 
the position of the high gravels with reference to the depressions occupied 
by the present streams. 
* See ante, p. 64. + See Appendix A, Section VII. 
