While Ocean wrap'd it in his azure robe.

CANTO II. l. 34.

From having observed that many of the highest mountains of the world consist of lime-stone replete with shells, and that these mountains bear the marks of having been lifted up by subterraneous fires from the interior parts of the globe; and as lime-stone replete with shells is found at the bottom of many of our deepest mines some philosophers have concluded that the nucleus of the earth was for many ages covered with water which was peopled with its adapted animals; that the shells and bones of these animals in a long series of time produced solid strata in the ocean surrounding the original nucleus.

These strata consist of the accumulated exuviae of shell-fish, the animals perished age after age but their shells remained, and in progression of time produced the amazing quantities of lime-stone which almost cover the earth. Other marine animals called coralloids raised walls and even mountains by the congeries of their calcareous habitations, these perpendicular corralline rocks make some parts of the Southern Ocean highly dangerous, as appears in the journals of Capt. Cook. From contemplating the immense strata of lime-stone, both in respect to their extent and thickness, formed from these shells of animals, philosophers have been led to conclude that much of the water of the sea has been converted into calcareous earth by passing through their organs of digestion. The formation of calcareous earth seems more particularly to be an animal process as the formation of clay belongs to the vegetable economy; thus the shells of crabs and other testaceous fish are annually reproduced from the mucous membrane beneath them; the shells of eggs are first a mucous membrane, and the calculi of the kidneys and those found in all other parts of our system which sometimes contain calcareous earth, seem to originate from inflamed membranes; the bones themselves consist of calcareous earth united with the phosphoric or animal acid, which may be separated by dissolving the ashes of calcined bones in the nitrous acid; the various secretions of animals, as their saliva and urine, abound likewise with calcareous earth, as appears by the incrustations about the teeth and the sediments of urine. It is probable that animal mucus is a previous process towards the formation of calcareous earth; and that all the calcareous earth in the world which is seen in lime-stones, marbles, spars, alabasters, marls, (which make up the greatest part of the earth's crust, as far as it has yet been penetrated,) have been formed originally by animal and vegetable bodies from the mass of water, and that by these means the solid part of the terraqueous globe has perpetually been in an increasing state and the water perpetually in a decreasing one.

After the mountains of shells and other recrements of aquatic animals were elevated above the water the upper heaps of them were gradually dissolved by rains and dews and oozing through were either perfectly crystallized in smaller cavities and formed calcareous spar, or were imperfectly crystallized on the roofs of larger cavities and produced stalactes; or mixing with other undissolved shells beneath them formed marbles, which were more or less crystallized and more or less pure; or lastly, after being dissolved, the water was exhaled from them in such a manner that the external parts became solid, and forming an arch prevented the internal parts from approaching each other so near as to become solid, and thus chalk was produced. I have specimens of chalk formed at the root of several stalactites, and in their central parts; and of other stalactites which are hollow like quills from a similar cause, viz. from the external part of the stalactite hardening first by its evaporation, and thus either attracting the internal dissolved particles to the crust, or preventing them from approaching each other so as to form a solid body. Of these I saw many hanging from the arched roof of a cellar under the high street in Edinburgh.

If this dissolved limestone met with vitriolic acid it was converted into alabaster, parting at the same time with its fixable air. If it met with the fluor acid it became fluor; if with the siliceous acid, flint; and when mixed with clay and sand, or either of them, acquires the name of marl. And under one or other of these forms composes a great part of the solid globe of the earth.

Another mode in which limestone appears is in the form of round granulated particles, but slightly cohering together; of this kind a bed extends over Lincoln heath, perhaps twenty miles long by ten wide. The form of this calcareous sand, its angles having been rubbed off, and the flatness of its bed, evinces that that part of the country was so formed under water, the particles of sand having thus been rounded, like all other rounded pebbles. This round form of calcareous sand and of other larger pebbles is produced under water, partly by their being more or less soluble in water, and hence the angular parts become dissolved, first, by their exposing a larger surface to the action of the menstruum, and secondly, from their attrition against each other by the streams or tides, for a great length of time, successively as they were collected, and perhaps when some of them had not acquired their hardest state.

This calcareous sand has generally been called ketton-stone and believed to resemble the spawn of fish, it has acquired a form so much rounder than siliceous sand from its being of so much softer a texture and also much more soluble in water. There are other soft calcareous stones called tupha which are deposited from water on mosses, as at Matlock, from which moss it is probable the water may receive something which induces it the readier to part with its earth.

In some lime-stones the living animals seem to have been buried as well as their shells during some great convulsion of nature, these shells contain a black coaly substance within them, in others some phlogiston or volatile alcali from the bodies of the dead animals remains mixed with the stone, which is then called liver-stone as it emits a sulphurous smell on being struck, and there is a stratum about six inches thick extends a considerable way over the iron ore at Wingerworth near Chesterfield in Derbyshire which seems evidently to have been formed from the shells of fresh-water muscles.

There is however another source of calcareous earth besides the aquatic one above described and that is from the recrements of land animals and vegetables as found in marls, which consist of various mixtures of calcareous earth, sand, and clay, all of them perhaps principally from vegetable origin.

Dr. Hutton is of opinion that the rocks of marble have been softened by fire into a fluid mass, which he thinks under immense pressure might be done without the escape of their carbonic acid or fixed air. Edinb. Transact. Vol. I. If this ingenious idea be allowed it might account for the purity of some white marbles, as during their fluid state there might be time for their partial impurities, whether from the bodies of the animals which produced the shells or from other extraneous matter, either to sublime to the uppermost part of the stratum or to subside to the lowermost part of it. As a confirmation of this theory of Dr. Hutton's it may be added that some calcareous stones are found mixed with lime, and have thence lost a part of their fixed air or carbonic gas, as the bath-stone, and on that account hardens on being exposed to the air, and mixed with sulphur produces calcareous liver of sulphur. Falconer on Bath-water. Vol. I. p. 156. and p. 257. Mr. Monnet found lime in powder in the mountains of Auvergne, and suspected it of volcanic origin. Kirwan's Min. p. 22.