DESCRIPTION ON PLATES. 183 
the living diatom, and it is only when the valves 
have been properly prepared by freeing them 
from all organic matter that they are distinctly 
seen. In the more difficult test-diatoms these 
markings are so minute that they are only 
brought into view by the use of wide-angled 
objectives of high power. and by skilful manipu- 
lation of the light, — oblique illumination. 
The mode of preparation for living diatoms con- 
sists in the destruction of the organic material — 
cell-wall and contents—by means of cineration 
or by boiling in strong nitric acid. In the fossil 
forms, this destruction of the organic portion of 
the little plant has been accomplished by “ the 
hand of time.” 
In consequence of this indestructible siliceous 
shell, a record is left to us of the diatomaceous 
flora of past ages, and while countless genera- 
tions of unicellular Algee, and of Infusoria which 
were associated with them in the ponds and 
streams of the past, have disappeared without 
leaving a trace of their existence, we have abun- 
dant material for the study of fossil diatoms. 
One of the most famous localities, both on ac- 
count of its extent and the variety of species 
found, is in the vicinity of Richmond, Va. This 
is a marine deposit which underlies the city, and 
extends for an unknown distance around it. The 
thickness of this deposit is said to be about 
eighteen feet. When we consider the minute 
size of these exquisite little natural gems, and 
the conditions under which they are formed, we 
