14 GEOLOGY. 
magnitude. Little vegetation covers the surface of their slopes, and their geological structure 
may be read at a glance, even at the distance of miles. 
I have mentioned as proofs of the different ages of this mountain system, and that which 
forms the coast line throughout the entire length of the State of California, that the Miocene 
Tertiaries, composing so large a part of the coast ranges, are here, above the foot-hills, entirely 
wanting. This is true, without exception, in all parts of the Peninsular mountains adjacent to 
the Fort Yuma and San Diego-trail; and, from the recorded observations of others, we learn 
that such is the case in all parts of the Sierra Nevada where these mountains have been crossed 
in the State of California. 3 
In 1855, when attached to the surveying party of Lieutenant. Williamson, United States 
Topographical Engineers, we crossed the Sierra Nevada from the head of the Sacramento 
valley to the valleys on Upper Pit river, without anywhere, after leaving the vicinity of Fort 
Reading, seeing traces of Miocene Tertiary rocks. In our subsequent examinations of the sum- 
mits and eastern slopes of the Cascade mountains, we met with no strata of that age till we 
had recrossed this mountain chain in Oregon, and descended into the Willamette valley. 
Throughout the whole length of the Sierra Nevada system, from the southern line of California 
to the Columbia, the Tertiaries of the coast cover its western base, but are nowhere, so far as 
at present known, found crowning its summits, or resting in its higher valleys. 
On the western border of the Colorado desert is a series of strata which contain fossils re- 
garded by Mr. Conrad as indicative of Miocene age ; and at least one species, the gigantic oyster, 
(Ostrea Titan,) characterisic of the Miocene beds of the coast. This formation skirts the east- 
ern bases of the Peninsular mountains in the same manner that those of San Diego border 
them on the west ; the strata in both cases being somewhat elevated and disturbed, yet reach- 
ing but little way up their sides, and being entirely wanting in the interval. These facts seem 
to prove conclusively that the Sierra Nevada was raised at a period considerably anterior to the 
elevation of the Coast mountains, and disprove the statement of a recent writer that the latter 
are the older.* This author, as was previously mentioned, assigns to the elevation of the coast 
range a period anterior to that of the Sierra Nevada, placing it at the end of the Eocene epoch. 
“he sedimentary rocks to which he ‘refers are doubtless of different ages. The white and 
red sandstones and conglomerate contain ro fossils, but probably should be included in the 
same category with the Miocene, Pliocene, and modern deposits which are found along the bases 
of the Sierra Nevada, where they have been elevated at a comparatively recent date. 
These strata furnish illustration of a truth of wide application, viz: that the elevation of most 
of our important mountain ranges cannot be referred to a single epoch, but has been due to a series of 
elevatory paroxysms, or to processes of upheaval, continuing to act through long periods of time. 
The metamorphic limestone, of which M. Marcou speaks, is a conspicuous feature in the geo- 
logical structure of most of the numerous ranges of the Sierra Nevada system. It is usually 
associated with metallic veins, and, for reasons that will be given more fully hereafter, I have 
referred it to the Carboniferous epoch. 
The discovery, by Mr. Blake, of strata in the Sierra Nevada containing Eocene fossils, proves 
the date of the principal elevation of that range to be subsequent to the Zocene. This fact, 
taken in connexion with the relations which the Sierra Nevada sustains to strata known to be 
of Miocene age, permits us to refer the principal elevatory paroxysm of this system to the in- 
terval between the Eocene and Miocene. 
: Such facts as have a bearing on the elevation of the Peninsular mountains, subsequent to the 
deposition of the Miocene strata, will best be communicated in connexion with the local 
. 
*Marcou’s Geology cf North America, p. 79. 
