Page:Popular Science Monthly Volume 8.djvu/187

Rh like a stone. This is especially the case with the Meandrina, or brain-stone coral, so named from the resemblance which its furrowed surface bears to the convoluted surface of the brain; hemispherical masses of this coral are not unfrequently to be seen in museums having a diameter of from two to three feet; and in the upraised coral-cliffs of Bermuda they are reported to be five or six feet in diameter. The polyps lie in rows along the furrowed surface, and the active life of the composite mass does not extend far down; its stony interior being the product of its earlier life, as the heart-wood of a tree is the product of previous successions of leaf-buds. But it is no more correct to say that the polyps have built up the stony mass, than it would be to say that the leaves of a tree build up its woody stem, or that our own soft parts build up our bony skeleton. The hard parts are formed in each case by a process of growth; soft tissue being first produced as a part of the animal body, and this being subsequently solidified by mineral deposit, the material for which is absorbed by the animal from the sea-water in which it lives.

The admirable researches of Mr. Darwin have shown us that, although the reef-building corals seem unable to live and grow at depths greater than twenty fathoms (one hundred and twenty feet), yet that if their base gradually subsides, at a rate not greater than that of coral-growth, the reef or island will be kept up to the surface by such growth; so that, if we could bore down into it, we might find the coral-structure to have a depth of many hundreds or even thousands of feet. The recent soundings of the Challenger around the Bermuda islands, which are entirely composed of coral, indicate that they form the summit of a pillar rising from a depth of twelve thousand feet; and as we have no instance of a mountain having such a shape, it seems probable that the upper part of this pillar, at any rate, must have been formed of coral, which kept growing upward, in the manner indicated by Mr. Darwin, while the bottom was slowly subsiding. It is commonly supposed by geologists that the limestone beds of which I have been speaking are the result of the metamorphosis of ancient coral formations, which attained their great thickness by continuous growth at their living surface, as their base gradually subsided. But it appears to me that all we know of existing coral formations renders it unlikely that there should have been such a continuity of area in ancient coral formations, as would be required to account for the continuity in the area of our great beds of carboniferous limestone; and that this continuity is far better accounted for by supposing them to have been formed in the manner I previously indicated—by the foraminiferal life which recent researches have shown to be even now forming a calcareous deposit over vast areas of the ocean-bottom.

Thus, then, we should regard the beds which show distinct coral-structure as representing reefs or islands of limited extent in the