A Manual of Elementary Geology / or, The Ancient Changes of the Earth and its Inhabitants as Illustrated by Geological Monuments

by Sir Charles Lyell

Fig. 16. Bacillaria vulgaris?

Fig. 17. Gaillonella distans.

Fig. 18. Gaillonella ferruginea.

These figures are magnified nearly 300 times, except the lower figure of G. ferruginea (fig. 18. a), which is magnified 2000 times.

Fragment of semi-opal from the great bed of Tripoli, Bilin.

Fig. 19. Natural size.

Fig. 20. The same magnified, showing circular articulations of a species of Gaillonella, and spiculæ of Spongilla.

The remains of these Diatomaceæ are of pure silex, and their forms are various, but very marked and constant in particular genera and species. Thus, in the family Bacillaria (see [fig. 16.]), the fossils preserved in tripoli are seen to exhibit the same divisions and transverse lines which characterize the living species of kindred form. With these, also, the siliceous spiculæ or internal supports of the freshwater sponge, or Spongilla of Lamarck, are sometimes intermingled (see the needle-shaped bodies in [fig. 20.]). These flinty cases and spiculæ, although hard, are very fragile, breaking like glass, and are therefore admirably adapted, when rubbed, for wearing down into a fine powder fit for polishing the surface of metals.