Geology. 269 
which represent a recent formation, in which you inform me the re- 
mains of the mammoth occur. The shells are generally those wae 
live in the adjacent waters, and indicate little if any change of te 
ature since their deposition, The littoral character of this formation is 
very evident. Water-worn shells and fragments show the action of the 
surf, whilst entire Specimens of wc and acl Pholadidee, and Saxicavee, 
and soon shores. se burrow in clay, mud or sand, beyond 
the ordinary action of the surf; whilst some are —— out by the 
tempest-driven surge, and others preyed upon b 1nd € 
cies of Virginia. This formation may, therefore, be regarded as of 
focene origin—an opinion in which I am confirmed by some fossils 
collected in California, by Dr. Heermann, consisting of decidedly Mio- 
cene forms; a Mercenaria, (M. perlaminosa,) Con., scarcely differing 
from a species of Cumberland county, N.J., ( . Ducatelii, Con.), a 
Cemoria, Pandora and Cardita of extinct species, ‘closely analogous to 
locene forms. I am inclined, also, to refer to this period a very dif- 
ferent group from Ocoya creek, the forms of which you sketched in 
Califor nia, as the specimens were too friable to be preserved. I do not 
> owesigr so recent species among them, nor any contained in an 
ocen epos 
The rock m San Diego is replete with shells, generally of a small 
size, and appears to have a certain paleontological relation to those of 
pad Coralia, and those from the rocks near Astoria in Oregon, 
collect ed by Townsend, and Dana, which I have referred to the Miocene 
period. Two species of San Diego, if not a rs yf Oregon 
shells; Nucula decisa is similar to N. divaricata, h, in their 
markings, resemble N. Cobboldii ns a i “aa Sisaitin. Mactra 
Diegoana is < apalt related to the Ore 
€ period is onequaalite: repoeaniod: ‘by - pre tt 
ou 
Eocene period. The vast distance between the two localities will ac- 
Count for the general distinction of species, and it was 2 as, indeed, an unex- 
= result to find any identical. If I me ape ed any ensiers 
