. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. I Mffll. a Bacillaria Gallonella Gallonella vulgaris ? distans. ferruginea. These figures are magnified nearly 800 times, except the lower figure of G. ferruginea (fig. 18 ay, which is magnified 2000 times. without any visible cement. It is difficult to convey an idea of their extreme minuteness; but Ehrenberg estimates that in the Bilin tripoli there are 41,000 millions of individuals of the Gaillonella distans (see fig. 17) in every cubic inch, which weighs about 2
. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. I Mffll. a Bacillaria Gallonella Gallonella vulgaris ? distans. ferruginea. These figures are magnified nearly 800 times, except the lower figure of G. ferruginea (fig. 18 ay, which is magnified 2000 times. without any visible cement. It is difficult to convey an idea of their extreme minuteness; but Ehrenberg estimates that in the Bilin tripoli there are 41,000 millions of individuals of the Gaillonella distans (see fig. 17) in every cubic inch, which weighs about 220 grains, or about 187 millions in a single grain. At every stroke, therefore, that we make with this polishing powder, several millions, perhaps tens of millions, of perfect fossils are crushed to atoms. The remains of these Diatomacese are of pure silex, and then- forms are various, but very marked and constant in particular genera and spe- Fig. 20. Fig. 19. cies. Thus, in the family Ba- cillaria (see fig. 16), the fos- sils preserved in tripoli are seen to exhibit the same di- visions and transverse lines which characterize the living species of kindred form. With these, also, the siliceous spicu- lae or internal supports of the freshwater sponge, or Sjwn- gilla of Lamarck, are some- times intermingled (see the needle-shaped bodies in fig. 20). These flinty cases and spicnke, although hard, are very fragile, breaking like glass, and are therefore admi- rably adapted, when rubbed, for wearing down into a fine powder fit for polishing the surface of metals. Fragment of semi-opal from the great bed of tripoli, Bilin. Besides the tripoli formed Fie. 19. Natural size. , . .. ' â *, I, , Fig. 20. The same magnified, showing circular articula- exclusively ot the IOSSllS above tions of a species of Gallonella, and spiculae -, m i xi, ' ⢠n of Sponqiiid. described, there occurs in the. Please note that these images are extracted from scanned page images that may have been digita
Size: 2381px × 1049px
Photo credit: © Paul Fearn / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1860, booksubjectgeology, bookyear1868
