4 THE AURIFEROUS GRAVELS OF THE SIERRA NEVADA. 
the aspect of a narrow lake. Reference will be made farther on to some of 
these points, in connection with remarks on the profile of the former and 
present river grades in the gravel region. 
The drainage into the Great Valley is very peculiar in character, and 
is the combined result of the climatic and topographical conditions of the 
region. This will be explained after a brief description of the chains of 
mountains which form the framework of the valley. 
The Coast Ranges form the limit of the Great Valley on the western side, 
and extend to the Pacific Ocean, there being nowhere on that side more than — 
a narrow space between the foot-hills and the ocean, while for a large part 
of the distance the steep slopes of the ranges come directly to the water’s edge. 
A glance at the “Map of California and Nevada” will show in a few moments, 
much better than could be explained in many words, the peculiar character 
of the topography of the Coast Ranges. The inosculation of the coast moun- 
tains with the Sierra Nevada at both ends of the Great Valley has already 
been alluded to. The general fact is at once recognized that the Coast 
Ranges are made up of numerous broken and often rather indistinct chains, 
which on the whole maintain a pretty well marked parallelism with the 
coast. This parallelism, however, is often better made out from an examina- 
tion of the courses of the rivers than from the position of the subordinate 
ranges. It will also be seen on the map, that while the coast mountains are 
often nearly broken through by cross fractures, giving a chance of escape for 
the secondary drainage, — that is, for the streams originating within the 
Coast Ranges themselves, — in only one place does the fracture or depression 
extend entirely across the whole series of chains. This takes place at the 
Bay of San Francisco, where is the only outlet for the entire drainage of the 
Great Valley. Here, in latitude 38°, the Sacramento and San Joaquin rivers 
unite in an extensive depression, partly occupied by low islands covered 
with a dense growth of “tule” (Scirpus palestris), and subject to overflow 
where not artificially protected, and partly by Suisun Bay and the Bays of 
San Francisco and San Pablo. Of these bays the two latter are in reality 
portions of one and the same thing, there being a narrow strait separating 
them, and they form a depression about fifty miles long, lying parallel with 
the general trend of the ranges, and enclosed within them ; while Suisun 
Bay, on the other hand, is rather at right engles to the others, trending across 
the ranges, and having no streams entering it, except the Sacramento and 
San Joaquin. It is, in fact, the half-submerged delta of these two rivers, and 
