Part 8 (2/2)

[Ill.u.s.tration: Fig. 12. Recent _Spatangus_ with the spines removed from one side.

_b._ Spine and tubercles, nat. size.

_a._ The same magnified.]

[Ill.u.s.tration: Fig. 13.

_a._ _Echinus_ from the chalk, with lower valve of the _Crania_ attached.

_b._ Upper valve of the _Crania_ detached.]

Now the series of events here attested by a single fossil may be carried a step farther. Thus, for example, we often meet with a sea-urchin in the chalk (see fig. 13.), which has fixed to it the lower valve of a _Crania_, a genus of bivalve mollusca. The upper valve (_b_, fig. 13.) is almost invariably wanting, though occasionally found in a perfect state of preservation in white chalk at some distance. In this case, we see clearly that the sea-urchin first lived from youth to age, then died and lost its spines, which were carried away. Then the young _Crania_ adhered to the bared sh.e.l.l, grew and perished in its turn; after which the upper valve was separated from the lower before the _Echinus_ became enveloped in chalky mud.

It may be well to mention one more ill.u.s.tration of the manner in which single fossils may sometimes throw light on a former state of things, both in the bed of the ocean and on some adjoining land. We meet with many fragments of wood bored by s.h.i.+p-worms at various depths in the clay on which London is built. Entire branches and stems of trees, several feet in length, are sometimes dug out, drilled all over by the holes of these borers, the tubes and sh.e.l.ls of the mollusk still remaining in the cylindrical hollows. In fig. 15. _e_, a representation is given of a piece of recent wood pierced by the _Teredo navalis_, or common s.h.i.+p-worm, which destroys wooden piles and s.h.i.+ps. When the cylindrical tube _d_ has been extracted from the wood, a sh.e.l.l is seen at the larger extremity, composed of two pieces, as shown at _c_. In like manner, a piece of fossil wood (_a_, fig. 14.) has been perforated by an animal of a kindred but extinct genus, called _Teredina_ by Lamarck. The calcareous tube of this mollusk was united and as it were soldered on to the valves of the sh.e.l.l (_b_), which therefore cannot be detached from the tube, like the valves of the recent _Teredo_. The wood in this fossil specimen is now converted into a stony ma.s.s, a mixture of clay and lime; but it must once have been buoyant and floating in the sea, when the _Teredinae_ lived upon it, perforating it in all directions. Again, before the infant colony settled upon the drift wood, the branch of a tree must have been floated down to the sea by a river, uprooted, perhaps, by a flood, or torn off and cast into the waves by the wind: and thus our thoughts are carried back to a prior period, when the tree grew for years on dry land, enjoying a fit soil and climate.

[2 Ill.u.s.trations: Fossil and recent wood drilled by perforating Mollusca.

Fig. 14. _a_. Fossil wood from London clay, bored by _Teredina_.

_b_. Sh.e.l.l and tube of _Teredina personata_, the right-hand figure the ventral, the left the dorsal view.

Fig. 15. _e_. Recent wood bored by _Teredo_.

_d_. Sh.e.l.l and tube of _Teredo navalis_, from the same.

_c_. Anterior and posterior view of the valves of same detached from the tube.]

It has been already remarked that there are rocks in the interior of continents, at various depths in the earth, and at great heights above the sea, almost entirely made up of the remains of zoophytes and testacea. Such ma.s.ses may be compared to modern oyster-beds and coral reefs; and, like them, the rate of increase must have been extremely gradual. But there are a variety of stony deposits in the earth's crust, now proved to have been derived from plants and animals, of which the organic origin was not suspected until of late years, even by naturalists. Great surprise was therefore created by the recent discovery of Professor Ehrenberg of Berlin, that a certain kind of siliceous stone, called tripoli, was entirely composed of millions of the remains of organic beings, which the Prussian naturalist refers to microscopic Infusoria, but which most others now believe to be plants.

They abound in freshwater lakes and ponds in England and other countries, and are termed Diatomaceae by those naturalists who believe in their vegetable origin. The substance alluded to has long been well known in the arts, being used in the form of powder for polis.h.i.+ng stones and metals. It has been procured, among other places, from Bilin, in Bohemia, where a single stratum, extending over a wide area, is no less than 14 feet thick. This stone, when examined with a powerful microscope, is found to consist of the siliceous plates or frustules of the above-mentioned Diatomaceae, united together 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 polis.h.i.+ng powder, several millions, perhaps tens of millions, of perfect fossils are crushed to atoms.

[3 Ill.u.s.trations: These figures are magnified nearly 300 times, except the lower figure of _G. ferruginea_ (fig. 18. _a_), which is magnified 2000 times.

Fig. 16. _Bacillaria vulgaris?_

Fig. 17. _Gaillonella distans._

Fig. 18. _Gaillonella ferruginea._]

[2 Ill.u.s.trations: 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 spiculae of _Spongilla_.]

The remains of these Diatomaceae 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 spiculae 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 spiculae, although hard, are very fragile, breaking like gla.s.s, and are therefore admirably adapted, when rubbed, for wearing down into a fine powder fit for polis.h.i.+ng the surface of metals.

Besides the tripoli, formed exclusively of the fossils above described, there occurs in the upper part of the great stratum at Bilin another heavier and more compact stone, a kind of semi-opal, in which innumerable parts of Diatomaceae and spiculae of the _Spongilla_ are filled with, and cemented together by, siliceous matter. It is supposed that the siliceous remains of the most delicate Diatomaceae have been dissolved by water, and have thus given rise to this opal in which the more durable fossils are preserved like insects in amber. This opinion is confirmed by the fact that the organic bodies decrease in number and sharpness of outline in proportion as the opaline cement increases in quant.i.ty.

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