DESMIDS AND DIATOMS. 593 
one epoch in the geological history of the Diatoms, and 
more attentive study will yet reveal the occurrence of 
similar special epochs in the case of other species, even 
though we may not be able to directly synchronize these 
epochs with those determined from other data.* But 
leaving the region of uncertainty, there are numerous 
deposits, the great antiquity of which is placed beyond a 
doubt. Among these we may first enumerate a deposit 
in which were found imbedded, in 1843, the bones of a 
Mastodon, in Orange county, N. Y., and which, from its 
peculiar connection with these bones, was undoubtedly of 
contemporaneous origin. Being unaffected by severity 
of climate, it is probable that the Diatoms continued to 
exist through the whole Post-tertiary Period, affording, 
by the entire absence of marine species, another confirm- 
ation of the much-disputed Glacier theory of Professor 
Agassiz. Again receding, the next deposits of which the 
age may be considered as definitely fixed, are those of 
Virginia and Maryland, the most celebrated of all diato- 
maceous earths, from the extreme variety and beauty of 
their forms and the extent of the beds containing them. 
These beds, where they underlie the city of Richmond, 
are not less than twenty feet in thickness, and consist 
entirely of marine remains; while deposits, similar in 
character, and probably contemporaneous in origin, are 
found at many localities as far as Piscataway, in the State 
of Maryland. They are referred by their discoverer, 
Professor W. B. Rogers, to the Miocene Tertiary. One 
cubic inch of the earth has been calculated to contain not 
less than several millions of individual shells. Many 
similar similar deposits have been observed both in America and 
uo a terstn Petal fos Lewis, in Er e Proceedings of the 
Philadelphia Academy of Natural 
AMERICAN NAT., VOL. I. 
