| Fig. | 1.— | Coprolite of a fish (Macropoma). Chalk, Lewes. |
| 2.— | Coprolite of a fish (Squalus). Chalk marl. Ramsey. | |
| 3.— | Molluskite of a Rostellaria. (Mr. Bensted.) Kentish Rag, Maidstone. |
This substance is of a dark brown or black colour, and occurs either in shapeless masses, which are irregularly distributed among the shells and other organic remains, in sandstone, limestone, &c., or as casts of shells, or occupying their cavities, as in the specimen [Lign. 139, fig. 3], which is a vertical section of a spiral univalve (Rostellaria), filled with the soft parts of the animal, converted into molluskite. Upon analysis this substance is found to contain a large proportion of animal carbon.[391] The rocks of firestone at Southbourne, on the Sussex coast, are mottled with brown molluskite and hard amorphous concretions, consisting of carbon and phosphate of lime, mixed with sand and other extraneous matter. Casts of shells, of the genera Venus, Arca, &c., entirely composed of the same kind of materials, are also abundant in those rocks. The lowermost bed of Galt, at its line of junction with the Greensand beneath, at Folkstone, and in many other localities, is largely composed of similar matter, resembling in appearance the fossils called Coprolites, hereafter described. The outer chamber of the Ammonites and other shells, so abundant in the Galt, are often filled with this substance. But the most interesting deposit of molluskite is in the Kentish Rag of Mr. Bensted's quarry, near Maidstone. This phenomenon had not escaped the notice of that intelligent and accurate observer, who liberally placed at my disposal numerous shells, particularly of Trigoniæ and Terebratulæ, which were filled with molluskite, and large slabs of the sandstone, full of concretionary and amorphous masses of the same. The latter, Mr. Bensted suggested, may have been derived from the soft bodies of the dead Mollusks, which, having become disengaged from their shells and aggregated together, had floated in the sea, until they became enveloped in the sand and mud, which have gradually consolidated into the arenaceous stone termed Kentish Rag, In illustration of this opinion, Mr. Bensted directed my attention to the following remarkable fact, related in the American Journal of Science:—In the year 1836, a fatal epidemic prevailed among the shell-fish of the Muskingum River, in the state of Ohio. It commenced in April, and continued until June, destroying millions of the mollusca that inhabited the beds of the tributary streams, and the river. As the animals died, the valves of the shells opened, and, decomposition commencing, the muscular adhesions gave way, and the fleshy portions rose to the surface of the water, leaving the shells in the bed of the river. As masses of the dead bodies floated down the current, the headlands of islands, piles of drifted wood, and the shores of the river, in many places, were covered with them; and the air in the vicinity was tainted with the putrid effluvium exhaling from these accumulations of decomposing animal matter. The cause of the epidemic was unknown.
[391] Some of this molluskite has, at my request, been analyzed by Mr. Rigg, who obliged me with the following remarks:—"After removing the lime by means of hydrochloric acid from ten grains of this substance, there remained 1.2 grain of dark powder, which gave, by analysis with oxide of copper, .16 of a cubic inch of carbonic acid, and a small portion of nitrogen. On subjecting to the same kind of analysis two grains of the darker body, without previously acting upon it by any acid, .054 of a cubic inch of carbonic acid was obtained; so that from these results there is no doubt but the darker portion of the molluskite contains about .35 per cent, of its weight of carbon in an organized state."
"Now nearly the whole of the shells in the beds of Kentish Rag," Mr. Bensted remarks, "have their shells open, as if they were dead before their envelopment in the deposit. And, from the large quantity of water-worn fragments of wood perforated by Pholades imbedded with them, it seems probable that this stratum had originally been a sand-bank covered with drifted wood and shells, thus presenting a very analogous condition to the phenomenon above described." The gelatinous bodies of the Trigoniæ, Ostreæ, Rostellariæ, Terebratulæ, &c., detached from their shells, may have been intermingled with the drifted wood in a sand-bank; while, in some instances, the animal matter would remain in the shells, be converted into molluskite, and retain the form of the original, as in the spiral univalve, represented in section, [Lign. 139, fig. 3].
A microscopical examination of the Maidstone molluskite detects, with a low power, innumerable portions of the nacreous laminæ of shells, intermingled with the carbonaceous matter, many siliceous spicula of Sponges, minute spines of Echinoderms, and fragments of Corals; these extraneous bodies probably became entangled among the floating animal matter. A large proportion of the shelly laminæ, examined with a high power, displays the peculiar structure of the Terebratulæ (see [Lign. 126, fig. 2a]), of which several species are abundant in the Kentish Rag.
The dark masses and veins so common in the Sussex and Purbeck marbles are produced by molluskite. If at the period of their envelopment the shells were empty, they became filled either with grey marl and limestone, or with white calcareous spar; but if they enclosed the bodies of the Mollusks, the soft mass was changed into carbonaceous matter; and in polished sections of the marble, the molluskite appears either in black or dark brown spots, or fills up the cavities of the shells. The dark blotches and veins observable in the fine pillars of Purbeck marble in the Temple Church, London, are produced by molluskite; and the most beautiful slabs of Sussex marble owe their appearance to the contrast produced by this black substance in contact with white calcareous spar.[392]
[392] See a "Memoir on the Carbonized Remains of Mollusca," by the author. Read before the Geological Society of London, February, 1843; and published in the American Journal of Science.
Carbon, resulting from animal remains, is of frequent occurrence in many strata; and the fetid emanations from certain limestones, upon being broken or rubbed, are attributable to the evolution of sulphuretted hydrogen, from the animal matter which they contain.