3dly, In the state of casts and impressions. Although in loose sand the shells are either empty, or filled with detritus easily removable by washing; in clay, limestone, and sandstone, the cavities are generally occupied by consolidated materials, which had entered when in a soft or fluid state; and frequently the substance of the shell has disappeared, and the stony cast of the interior alone remains. In many instances, the spaces left by the dissolution of the shells are filled with spar, or the casts are closely invested by the surrounding stone, from long-continued superincumbent pressure while the matrix was in a plastic state; and in such cases the casts are often distorted and flattened. But the vacancy is occasionally empty, and on its walls is found an impress of the external surface of the shell, with all the lines and ornaments of the original as sharp as if cast in plaster of Paris.
Lign. 122. Turritellæ, from Bracklesham, Sussex. Tert.
| Fig. | 1.— | Turritella conoidea; the perfect shell: nat. |
| 2.— | Septarium, with Turritellæ; a polished slab: 1/3 nat. | |
| 3.— | A cast of one of the shells, in calcareous spar: nat. |
SEPTARIUM WITH SHELLS.
The specimen, [Lign. 122, fig. 2], from the tertiary strata at Bracklesham Bay, Sussex, is a polished slice of indurated argillaceous limestone, from a septarium (nodule divided by fissures), abounding in spiral univalve shells, called Turritellæ. Fig. 1 is a perfect shell of the same species, extracted from soft clay; and fig. 3, a cast in calcareous spar, obtained from the septarium. In the polished slab, fig. 2, sections of numerous shells are seen. The dark partitions, or septa, are veins of spar, which occupy interstices that have been formed in the clay-nodule by shrinking; and if the specimen be closely examined, the shells will be found split across and displaced by the fissures; thus presenting an interesting illustration of the faults, or dislocations, of the strata, so familiar to the geological observer. In the present instance, the lines on the exterior of the shell do not materially differ from those on the interior, and, consequently, the cast, fig. 3, and the shell. fig. 1, resemble each other; but in many species there is a striking contrast between the outer and inner surfaces, the external aspect being strongly ornamented, while the internal is smooth; the cast, therefore, in such examples, so little resembles the shell, that an inexperienced collector may readily suppose it belongs to a different species. The bivalve called Trigonia, [Lign. 127, figs. 1, 2], is an instance of this contrast.
The polished slab of the Septarium, [Lign. 122, fig. 2], demonstrates another condition of fossil shells—that of a compact argillaceous limestone—and entire beds of marble are composed of an aggregation of this kind, formed of shells and other animal exuviæ, consolidated by mineral infiltrations. In the older secondary strata this state prevails; and the beautiful markings of many valuable marbles, are merely sections of the enclosed shells. But this process is not restricted to the deposits of ancient date; at the present moment the same operation is silently but constantly going on in our seas, and an examination of the specimen, Lign. 123, will afford an exemplification of the manner in which these shelly limestones are produced.