58 
HISTOLOGY OF VEGETABLES. 
especially in America, there are strata of infusorial 
remains, many miles in extent. The city of Richmond, 
in Virginia, is built upon a stratum eighteen feet deep, 
FIG. 43 . 
* c -D 
a, Synedra. b, Navicula grandis. c, Isthmia obliquata. d, Campylo. 
discus clypeus. e, Eunotia. f g, Gallionella sulcata, h, Zygoceros rhom- 
bus. i, Dictyocha fibula. k, Cocconeis. 
and upwards of thirty miles in length, which is shown by 
the microscope to consist of little else than well-marked 
infusorial remains, some of which are represented by E, f, 
g, h, i, k, in Fig. 43. 
These minute creatures once lived in the water, from 
which they had the power of separating the silica, in the 
same manner as their representatives of the present 
