Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/31

18 18 FORMATION OF BOCKS IN SOUTH WALES

chiefly, if not altogether, extracted by the stone-making polyps from the adjacent water. There seems good reason for believing that there are very great beds of the shells of molluscs in many places, forming scarcely less important accumulations of the matter of limestone. Some oceanic islands to which the drift of common mud and sand cannot get, seem composed of little else than the calcareous matter which has once formed the harder parts of animals, such, for instance, as the Bermudas.*

In situations where calcareous matter is thus accumulating it is not difficult to see that the carbonic acid produced by the decomposition of the animal matter, in those cases where the animals perish naturally and are not eaten, but left to decompose, would, when confined in cavities, such as are common in a coral reef, act upon the carbonate of lime formed by the animals, thus furnishing the substance needftd for its solution and which may be readily again driven oS, aUowing a purely chemical deposit of carbonate of lime to be effected, either in the cavities of such coral ree& or in adjacent situations, and connecting in one mass carbonate of lime, in forms such as it was produced through the agency of animal life, with carbonate of lime obtained by ordinary chemical means.

On some oceanic coasts the sands consist, to a very great extent of pounded and broken shells, so that these sands, often employed on our own coasts for agricultural purposes, sometimes contain 70 per cent, of carbonate of lime. When driven on shore, as they sometimes are, by the united action of the sea and wind, they are well known to become consolidated as we should expect they would be, atmospheric water, sometimes percolating among vegetation, dissolving enough of the grains to cement the mass together. So firm is the resulting rock, that when a mass of it is struck, pebbles of quartz will sometimes be divided by a blow in the middle of this consolidated sand.

Where such sands are found on the shores we expect and usually find the adjacent sea bottoms composed of the same materials, which are nothing more than the remains of multitudes of molluscs and other marine animals having harder parts composed of carbonate of lime, to which the animals living in and upon these remains will again make ad- ditions.

From deposits and accumulations of the kinds mentioned, greater or less areas can be covered by nearly pure calcareous matter, and some of them may be mixed in many a comparatively tranquil situation with ordinary mud or clay ; also finding rest from mechanical suspension in

is gtTen by Lieutenant Nelson, of the Royal Engineers, in VoL T., Second Series, of the Transactions of the Geological Society of London. Most of the facts therein noticed are of great yalue, when we compare the limestones of farious geological dates with the limestone formations of the present day.
 * A yery Taloable and interesting account of the geological struetiire of the Bermudas

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