594 DESMIDS AND DIATOMS. 
Europe, but little has as yet been done in determining 
their precise age, or in accounting for the conditions 
necessary for the local accumulation of such vast quanti- 
ties of material. Among the most remarkable in this 
respect are those of our western coast. I have now before 
me a block of pure diatomaceous earth, a foot and a half 
long by half a foot in depth, of chalk-like whiteness, sent 
by Mr. W. P. Blake, from Monterey (the entire weight 
of which is only about six pounds), and other similar 
beds are found at many points in Mexico, California, and 
Oregon. One of these, discovered by Colonel Fremont 
on the Columbia River, surpasses all other known depos- 
its, being not less than 500 feet in thickness, and covered 
by at least 100 feet of compact basalt and other volcanic 
products ! 
It is probable that the Mexican and California beds, 
like those of Richmond, are of Tertiary age, though some 
of them may prove to be Cretaceous. That those of 
Monterey and San Francisco are far more ancient than 
the present physical features of California is proved by 
their being purely marine deposits, and by their differing 
wholly in character and species from other deposits, also 
of considerable thickness, from the eastern side of the 
Sierras, which I have lately had an opportunity of exam- 
ining. These latter are fluviatile or lacustrine, and con- 
tain many- species identical with those of the ordinary 
subpeat deposits of the Eastern States. 
In passing from the Tertiary to earlier formations, the 
evidence of the existence of the microscopic Alge be- — 
comes less evident, and for a long time none were 
believed to exist of more ancient date than those above 
alluded to. Certain peculiar organisms termed Xanthidia 
were, however, observed as of frequent occurrence in 
