GEOLOGY OF THE SIERRA NEVADA. 45 
rare everywhere except in the slates which are evidently the natural “hab- 
itat”’ of the gold. 
Just at the southern line of Mariposa County the auriferous belt widens 
out suddenly, and the formation becomes of importance. To the south of 
this, along the foot-hills of the Sierra, the metamorphic rocks can hardly be 
said to exist at all, the granite coming almost down to the plain. The outer 
edge of the granite through Fresno County is far from homogeneous in char- 
acter; it resembles something half-way between granite and gneiss, and 
contains frequent segregations of the minerals occurring most commonly in 
the metamorphic rocks, such as epidote and hornblende. It is lithologically 
such a formation as might have been expected to result from intense metamor- 
phic action along the edge of the granitic and the sedimentary masses. Pass- 
ing into Mariposa County, we find the auriferous slate series becoming, all 
at once, quite well developed, and from this county north as far as Plumas, 
the western slope of the Sierra is the field of active mining operations, while 
but comparatively little of value has been found anywhere to the south of 
this. From the Mariposa Estate north, for a distance of about a hundred 
miles along the flanks of the Sierra, the order of succession of the metamor- 
phic rocks — the auriferous belt, so called — is decidedly less obscure than it 
is beyond that point, and some subdivisions of the series can be traced for a 
considerable distance with the same lithological characters. In the region 
to the north of this, and especially in Sierra and Plumas counties, however, 
not only do we find the rocks so broken up that no one kind can be traced 
for any considerable distance ; but they are also very largely covered by 
eruptive materials, so that, even if there were some regular order of suc- 
cession existing, it would be extremely difficult to make it out, at least with- 
out much more detailed observations made with the aid of accurate maps on 
a large scale. The condition of things, in regard to solving the geological 
problems here presented, is in many respects like that existing in New 
England, where extreme scarcity of fossils and complete metamorphism of 
the strata have combined to render the task of working out the structural 
geology so difficult, that, thus far, in spite of many years of exploration, by 
far the larger part still remains to be done. And in New England the 
“hbed-rock” is covered much less deeply with detrital materials than it is in 
the Sierra Nevada. 
To give an idea of the distribution of the rocks belonging to the bed-rock 
series in that portion of the auriferous belt where they are least broken, and 
