The extensive beds of coralline limestones, which are found in the Silurian strata, wherever they occur,—for the limestones of this system in North America are characterized by the same species of corals as those of England,—seem to indicate that a more equal temperature prevailed throughout the ocean, at that geological epoch, than at the present time, when the geographical distribution of the coral zoophytes is strictly limited by temperature. The reef-forming genera are now confined to waters where the temperature is not below 70°; their most prolific development being 76°. The apparent exception, the occurrence of coral-reefs at the Bermudas, is found to depend upon proximity to the Gulf Stream (Wond. p. 614), which brings down the thermal waters of the tropics, and increases the local temperature of the sea in those localities.[259]
[259] Mr. Deane's splendid and masterly work on Corals, should be studied by those who wish to be acquainted with the present state of this branch of natural history.
ON COLLECTING FOSSIL CORALS.
On Collecting Fossil Corals.—Few instructions are required for the collection of fossil zoophytes; for as the most important characters of the several kinds have been pointed out in the previous descriptions, the student will be able to select illustrative specimens for his cabinet. The minute corals, &c. of the Chalk, and other limestones, are to be obtained by the same process as that directed for the discovery and preservation of the foraminifera, and other microscopic organisms, at the end of the next chapter. The larger examples should be left attached to a piece of chalk, when practicable, and the surrounding stone removed with a knife or graver, so as to expose as much of the fossil as may be required for the display of its characters, without loosening its attachment to the block. When the investing chalk is very hard, frequently pencilling the specimen with vinegar, or dilute hydrochloric acid, will soften the stone, and render its removal easy, by means of a soft brush: when acid is employed, the specimen must afterwards be well rinsed in cold water.[260]
[260] It may be well to caution the collector against employing sulphuric acid (commonly called oil of vitriol) for this purpose, for a white insoluble deposit (sulphate of lime) will thus be formed on the specimen, and its appearance irremediably injured. Many of the fossil corals obtained from the chalk of Dover Cliffs, are so saturated with muriate of soda, from long exposure to the spray of the sea, as to be liable to decomposition in the course of a few weeks, and are therefore not worth purchasing of the dealers.
The zoophytes that are in part flint, and part chalk, as the Ventriculites, (ante, [p. 244], [Lign. 81],) can rarely be obtained, except through the quarrymen who have been instructed how to extract them from the rock. The first specimen of this kind that came under my notice, I discovered while breaking a mass of chalk, in search of fungiform flints; when, to my great delight, I found the fossil, [Lign. 81], fig. 3, by which at once, and for the first time, was shown the connexion between the chalk specimens, [Lign. 8], figs. 1, 2, and the flints figured in [Lign. 8], figs. 2, 3, 6, 7, 8. Upon showing this fossil to the quarrymen, and exciting their attention by suitable rewards, I obtained the illustrative series now in the British Museum.[261] Much light would be thrown on the nature of other zoophytes of the chalk that are invested with flint, if due care were taken in the collection of specimens, and they were examined before extracted from the rock. Loose, delicate specimens, whether from the chalk or tertiary strata, should be affixed with strong gum-water to cards, or pieces of thin board, covered with coloured paper.
[261] Petrifactions, Room VI. p. 466.
The Greensand Spongites, Siphoniæ, &c. may often be extracted from the rock tolerably perfect, by a well-directed blow of the hammer; but fragile species should be left attached to a block, and the surrounding stone be carefully chiselled away, so as to expose the most essential characters.
The Faringdon zoophytes are, for the most part, encrusted by an aggregation of minute polyparia, shells, and detritus, which may be partially removed by washing with a stiff brush, and their cavities cleared with a stout penknife, removing the extraneous matter by chipping, not by scraping, or the surface will be injured. In this manner the beautiful specimens figured, ante, [p. 228]., were developed.
The Corals in the hard limestones can seldom be chiselled out to advantage; for the most part, polished sections best exhibit the form and structure of the originals.