176 
GEOLOGY. 
larger than any in the bed. This is also new, and is named 0. Heermani. It is about six inches 
in length. Mr. Conrad notes an analogy between these fossils and those of the Miocene strata 
of Virginia. The Anomia is allied to A. Buffini, and Ostrea vespertina to 0. subfalcata. He 
is inclined to regard the formation as Miocene. The Miocene fossils, to which he refers in con¬ 
nexion with these, in his letter, were from a different locality, being from the coast, at San 
Diego or Santa Barbara. 
SAN DIEGO. 
The gradual slope extending from the base of the Peninsula Sierra to the sea at San Diego 
appears to consist of nearly horizontal, or hut gently inclined, Tertiary strata. Its upper mar¬ 
gin, at the foot of the ridges, has a general elevation of about 1,200 feet, and its descent to the 
sea, fifteen miles distant, is very gradual. The streams in their descent cut canal-like valleys, 
with steep sides. These are very narrow, and are generally invisible from a short distance— 
the general aspect of the slope being that of an unbroken plain. The surface, however, and the 
sides of the valleys hear no resemblance to the barren and arid exposures of the formations of 
Carrizo Creek, on the other side of the mountains. On the seaward side, the surface is soil- 
covered and obscured by vegetation, and the edges of the strata along the valleys of erosion 
are hid by a talus of debris and earth. It therefore was not possible to determine the litholo¬ 
gical characters of the strata with any detail. In descending the mountains from San Pasqual 
towards San Diego, rounded hills were observed just before reaching the slope. These appeared 
to he formed of horizontal strata, and were of light argillaceous sand and the debris of granite. 
Their relations to the strata of the adjoining slope were not determined, hut they were supposed 
to he a part of the same series. On the road from the Missioh to “ Old Town,” or San Diego, 
a slight exposure of the strata at the road-side consists of soft sandstone and clay ; principally 
friable, argillaceous sandstone, colored dark brown by bitumen or coaly matter. The presence 
of coal or lignite in the vicinity of San Diego has been several times reported, hut no specimens 
have come under my observation. 
On the left or north side of the entrance to the bay, there is a series of rounded hills rising 
to the elevation of 200 or 400 feet. They appear to consist entirely of soft and semi-consolidated 
strata ; and at the end of Point Loma, where they have been undermined by the surf, are com¬ 
posed of sand and pebbles, with the lines of stratification slightly inclined inland. It is doubt¬ 
ful whether these strata are an upraised portion of those forming the slope or a more modern 
deposit. A heavy dyke of greenstone extends parallel with the foot of the mountains, and at 
some points may have uplifted and broken the strata, but there was no good evidence of it along 
the road. A limited exposure of sandstone, in the hanks of a small brook not far from this 
dyke, consisted of beds inclined at an angle of less than ten degrees. These were surmounted 
by a conglomerate of rounded pebbles like the shingle of a beach ; similar accumulations were 
noted at other points on the surface of the slope. 
Fossils. —Before leaving the Mission of San Diego, a block of sandstone, filled with fossils, 
was handed to me, but the locality was not seen. It is a compact sandstone, not unlike that of 
the Bay of San Francisco and Oregon. Mr. Conrad finds it to contain the following species: 
Cardium modestum , Nucula decisa , Corbula Diegoana , Mactra Diegoana , Natica Diegocma, Tro- 
chita Diegoana , Tellina Diegoana , and T. congesta . He also remarks a palaeontological relation 
between these fossils and those of Monterey, Carmello, and those found in boulders in Oregon 
by Mr. Townsend and Professor Dana. It is indeed uncertain whether two of the San Diego 
