146 Scientific Intelligence. 
on the west side; while north of American River they constitute 
nearly the whole width, only occasional areas of granite occurrin 
along the crest and on the eastern slope; and little of the granite 
gneissoid. 
7) Its outflow of igneous rocks, which spread over the gravel 
deposits of much of the western slope in the several counties from 
Mariposa to Plumas, increase in extent in Butte and Plumas 
Counties, and cover nearly the whole of the surface north of 
southward to that where granite is the chief rock. El Dorado, 
Placer and Nevada are the great mining counties. In the granitic 
and extensively worn away by erosion.” At present, the rivers 
and gorges are cut to a depth of many hundreds of feet below 
the surface either side. Starting from the foot-hills, the level of 
the surface between the streams rises much faster than the beds 
of the streams, so that when up 3,000 to 4,000 feet, the cuts are 
in many cases 1,000 to 2,000 feet deep. Further, there is a 
great difference between the level of the present beds and those 
of the era in which the gravels were deposited ; this difference 
being 1,800 feet in the Middle Fork of American River at Michigan 
Bluff, 1,300 feet at Iowa Hill, on the divide south of the North 
Fork of the American. These cafions increase in depth to the 
north of the South Yuba: the difference of level between the 
Middle Fork of Feather River at Nelson’s Point and the summit 
of the lava-bed of Pilot Peak which overlies the gravel being 
fully 3,650 feet; between the top of Mount Clermont, which is 
topped with gravel, and the valley at its base, 3,750 feet ; between 
the top of Spahish Peak, which is capped with lava and gravel, 
and the valley of American River, 3,800 feet. 
“An excellent idea of the topography of the hydraulic mining 
egion is got by the traveler passing over the line of the Central 
Pacific Railroad, in descending the slope of the Sierra. After 
passing Blue Cafion, the slates begin to be met with, and all 
along below this, especially in the neighborhood of Dutch Flat, 
and b t for several miles, the road passes through 2 
region of hydraulic mines, keeping on what seems to be a broad 
