TERTIARY STRATA ON SAN DIEGO RIVER. 15 
VICINITY OF SAN DIEGO. 
The traveller entering the harbor of San Diego sees before him a long line of table-land, of 
which the edge presents a somewhat abrupt declivity, preserving a general parallelism with 
the line of the beach ; its summit elevated from two to three hundred feet above the present 
level of the ocean; the entire landscape devoid of trees ; the contour of the surface, visible 
for miles, everywhere exhibiting a picture of monotonous barrenness. 
This table-land extends, without interruption and with a very gentle ascent, to the foot-hills 
of the Peninsular mountains, a distance of from twelve to fifteen miles. Through this plateau 
the San Diego river has eroded a narrow and winding valley, from the point where it issues 
from the hills till it reaches the ocean. On the sides of this valley the cut edges of the strata 
composing the table-land are very fully exposed. They consist of soft, yellowish sandstones ; 
white, chalk-like, infusorial earths, conglomerates and clays ; and include concretions and thin 
bands of limestone, in great part composed of shells, which are frequently preserved in a 
beautiful and perfect condition. The best exposure of these fossiliferous beds is across the 
valley immediately opposite the mission of San Diego, and from this locality I collected a large 
number of species of both bivalve and univalve shells, and a single echinoderm. Some of these 
species are new to science. but many are identical with those found in the fossiliferous portions 
of the Miocene series further up the coast. 
The rocks represented in this section are probably equivalents of the middle and some part 
of the upper portion of the series exposed on the shore of San Pablo bay. They overlie the 
massive non-fossiliferous sandstones and shales of San Francisco, and are doubtless of the 
same age with a portion, at least, of the infusorial and fossiliferous deposits of Monterey, Santa 
Barbara, &c. In looking up the valley of the San Diego river the outcrop of a single stratum 
may be traced for several miles. The beds are thus seen to be entirely undisturbed, all gently 
dipping towards the ocean. : 
Following up the San Diego river, about five miles above the mission, we find it issuing from 
& Narrow cafion cut in a mass of augitic trap, which forms the first of the series of ranges com- 
posing the Peninsular mountains. By the protrusion of this trap the Tertiary strata have been 
somewhat elevated and disturbed, though in no considerable degree metamorphosed. I am 
inclined to regard the trap exposed at this point as much more modern than the great mass of 
the mountain system with which it is connected, and its eruption as marking a paroxysm of 
elevation long subsequent to the upheaval of the chain. The existence of masses of modern 
trap along the bases of the different ranges of the Sierra Nevada system—a fact to which I 
shall have occasion to refer repeatedly in treating of the geology of the banks of the Colorado— 
has seemed to me of especial interest, as showing the continued and comparatively recent 
action of disturbing and elevating forces along these ancient lines of upheaval. 
SAN DIEGO TO THE DESERT. 
A mass of trap, similar to that just described, is scen in passing from San Diego to San 
Pasqual, near Penasquitas. The Tertiary strata are there upheaved by it, but are not again 
seen, going eastward. From San Pasqual the trail passes over a high ridge, from which the 
ocean is distinctly visible. This ridge is composed of gray felspathic granite, traversed by 
veins of felspar of great size. These often include large quantities of schorl, frequently 
exhibiting handsome crystallizations. 
On the east side of San Pasqual mountain we descended to Laguna, a valley on the Pacific 
side of the main divide, and drained through a cafion cut in a portion of the San Pasqual 
mountain. From Laguna, continuing eastward, the road follows up a valley to the third range 
