Page:Hardwicke's Science-Gossip - Volume 1.pdf/238

much-disputed point. By some, especially by the earlier authorities, it was thought to be a very slow process, and it was supposed that, on the average, little more than a foot could be added to a reef during a whole century. But this cannot always be the case, for Darwin speaks of a ship-bottom which was covered to the thickness of two feet in twenty months. Loose masses also have been known to become firmly cemented in six months by the growth of new coral. A further instance of the rapidity of growth was found in the Keeling Reef, a channel in which became entirely stopped up, through which a schooner had floated only ten years before.

From this fact Darwin draws the following conclusion:—"First, that considerable thickness of rock has certainly been formed within the present geological era by the growth of coral, and by the accumulation of its detritus; and, secondly, that the increase of individual corals and reefs, both outwards or horizontally, and upwards or vertically, under peculiar conditions favourable to such increase, is not slow when referred either to the standard of the average oscillations of level in the earth's crust, or to the more precise but less important one of a cycle of years."

Coral reefs, partaking as they do of the depression or elevation of the sea-bottom, and also being subject to the wares and breakers, form a barrier of limestone more or less compact; and as the polype ceases on reaching the surface of the water, the top of the reef frequently becomes weathered and converted into soil capable of sustaining vegetation. When the sea-bottom to which the zoophytes are attached partakes of a gradual elevation, they build outwards and seawards, and should it be undergoing depression, they strike upwards.

These sea mountains abound in the Pacific, Indian, and Southern Oceans. Masses of them abound in the Pacific, on both sides of the equator, but not beyond the 30th degree of latitude. They are also very numerous in the southern part of the Indian Ocean. For hundreds of miles, we find them trending along the north-east coast of Australia, and they occur more or less in the Persian, Arabian, Red, and Mediterranean Seas.

Owing to the great amount of volcanic agency which is constantly at work, upheaving and submerging in the Pacific, there are found there many peculiar phases of the coral reef; such, for instance, as the atolls or lagoon islands, fringing or shore reefs, coral ledges, and encircling reefs.

Atolls consist of coral reefs forming low circular slands, enclosing lagoons. Shore reefs are those which surround islands of igneous and other origin. Coral ledges are masses of coral thrown up by volcanic agency on to the top of other reefs already upheaved. The encircling reef is that which stretches along shore in surf-beaten ridges, often extending for many leagues, and varying in thickness from 20 to 200 feet. The great reef which lies off the coast of New Holland is described by Captain Flinders as being more than 1,000 miles long, and varying in thickness from 20 to 100 feet.

In this same reef there is one continuous portion of more than 350 miles, without a single break or passage through it.

So much for coral reefs and islands, commonly so called. But modern discoveries go to show that the work of the polype is not confined to those mountains which are still covered by the sea. On the contrary, it is, I believe, the general belief among the great geologists of our day, that the Dolomite mountains, hitherto the cause of so much doubtful controversy, were the work of the coral zoophyte.

These stratified rocks, deeply divided by vertical fissures, and scattered about over porphyritic platforms, with so little connection with the neighbouring rocks, that they look like "icebergs stranded," have been shown by Dr. Richthofen, Mr. Churchill, &c., to bear a most remarkable resemblance to coral reefs.

And here I cannot do better than quote some passages from a review on Gilbert and Churchill's book on "The Dolomite Mountains:"—

"Mr. Churchill suggests that the formation of Dolomite may be going on in coral reefs at the present day; for the specimens of coral rock brought home by Dana from the raised island of Mantea, or Aurora, were found to contain in one case 5 per cent. and in another as much as 38 per cent. of carbonate of magnesia. And if we suppose the Dolomite mountains of Carinthia to have been formed on a gradually subsiding basis, they may have grown up like the low islands of the Pacific, till the sea attained the depth of a thousand fathoms, preserving their original contour from first to last, the group of corals, like a forest of tree trunks without tops, rising upwards together, and becoming partially solid by lateral growth, or by filling up with sediment.

"We have no fossil coral in England wherewith to compare the Dolomite mountains. Our magnesian limestone affords only bryozoa, for it has not been suspected that the remarkably concentric and radiated concretions are metamorphosed corals. In one Silurian coral reef of the Wenlock Edge and Dudley there may be masses of branching coral a yard across, and convex Stromatoporæ (which are not corals) of nearly equal size. But the coral beds are separated by clay partings, and never attain a great thickness. The Devonshire marbles have much the appearance of coral reefs, so far as respects the scattering of small masses over a region of argillaceous schists. In the carboniferous limestone layer above layer of branching corals may be seen in the lofty cliffs of Cheddar and the weather-beaten shores of Lough Erne. There the corals are slightly