304 RESUME AND THEORETICAL DISCUSSION. 
When we pass the North Fork of the American we come into a region 
where not only is the gravel most heavily developed, but where consid- 
erable areas of it are uncovered by the volcanic capping. An inspection of 
the General Gravel Map, which embraces the region between the Middle 
Fork of the American and the Middle Yuba, will show this fact very clearly.* 
Relatively to the whole area of the country, the gravel occupies more space, 
and the voleanic formations less, to the north of the North Fork than they 
do to the south of that stream. The lava-flows reach from high up in the 
Sierra, in almost unbroken lines, along the summits of the flat-topped ridges 
which occur between the cafons on the head-waters of Bear River; but the 
workable deposits of gravel are accumulated chiefly along a line nearly par- 
allel with the crest of the Sierra, in great, uncovered masses, which extend 
in irregular but almost unbroken succession from Indiana Hill to beyond 
Quaker Hill. These great areas of uncovered gravel, as described by Pro- 
fessor Pettee in the preceding pages ¢ and in Appendix A, lie at very nearly 
the same elevation, namely, a little more or a little less than 3,000 feet, the 
bed-rock at Indiana Hill being about 2,800 feet, and that at Quaker Hill 
from 2,700 to 3,000 feet in elevation, while the height of the summit of the 
gravel banks at Plug Ugly Hill is 3,251 feet, and at Quaker Hill 3,130 feet. 
The thickness of these gravel deposits is also very considerable, — perhaps 
greater than anywhere else in the State, the fact being taken into considera- 
tion that most of it is unmixed with volcanic débris. Thus, at Indiana Hill 
there is a thickness of 400 feet of clean, almost uniform gravel, with hardly 
any admixture of either clay or sand. 
Between Greenhorn Creek, the most northern branch of Bear River, and 
Deer Creek the gravels are continuous at a high altitude from Bunker Hill 
to Scott’s Flat, forming the continuation of the great uncovered area just 
indicated as extending north from Indiana Hill to Quaker Hill. But below 
these there are also other gravels, of less importance, it is true,which lie around 
Grass Valley and Nevada City, and not at a much lower level than those just 
described. These deposits are of considerable thickness at a few points, and 
in places must have been fairly rich in gold; but they have never had any- 
thing like the importance of those farther north on the branches of the Yuba, 
or south on those of the American. 
If the gravels at the head of Bear River present what may be called the 
* See also the detailed map, on a much larger scale, of the Dutch Flat Region. 
t+ See ante, pp. 143-160. 
