Page:Popular Science Monthly Volume 8.djvu/186

174 remains abundant in specimens of this limestone which do not show any indications of organic structure that are obvious to the naked eye. If the Globigerina-mud were to be subjected to the pressure of an enormous weight of rock deposited above it, and then to the heat and pressure which we know must have accompanied the great crumpling of the earth's crust that made the marked separation between the Paleozoic and the Secondary epochs, we may well believe that it would have been metamorphosed into a limestone closely resembling the least fossiliferous of the Avonside rocks; and we have no difficulty in accounting for the vast thickness of these beds, if we regard them as having been progressively formed on the bottom of a very deep ocean, through a long succession of ages.

That certain beds of the Avonside rocks are ancient Coral-Reefs, cannot be a matter of question; for we find them to be entirely made up of fossil corals, together with the fossilized shells and crinoids which such reefs would have supported. This was especially the case with what used to be called the "black rock" under the seawall, which has been nearly all quarried away since, when a boy, I brought home a piece of it as large as I could carry, wondering at such an accumulation of fossils, but without any such understanding of their import as that which I am endeavoring to give you. Every one has heard of the coral reefs and islands, which are popularly said to be "built up" in tropical seas by the agency of "insects," as bees build their waxen combs. And I suppose that every one of you is familiar with specimens of some kind of coral brought home by a seafaring friend, or has seen such in your museum. Now, the fact is, that all these corals are the production of animals resembling in essential points the common sea-anemone, but differing from it in depositing a stony skeleton in the fleshy substance which forms its base, and also in the radiating partitions which surround its stomach. We have on our own shores a small type of the coral-forming polyps, in the little Caryophyllla, which, when the animal is expanded, you would take to be a small sea-anemone, but which, when contracted, shrinks down into its stony cup. The Fungia of tropical seas is a much larger solitary polyp of the same kind; and you will often meet with its stony disk, four or five inches in diameter, with beautiful thin vertical plates radiating from the centre to the circumference, very much like the "gills" of the under-side of a mushroom (fungus), whence its name is derived. But all the more massive corals are the skeletons of composite animals; that is, of polyps which bud like plants, and thus grow to large dimensions. In some cases they form tree-like structures, in which you will find a multitude of polyp-cells, sometimes very small, each having its characteristic arrangement of radiating plates. But in the reef-building corals, the polyp-cells are packed closely together; and the older portion becomes so completely solidified by calcareous deposit that, when broken across, it looks