. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. Phasianella Heddingtonensis, and cast of the same. Coral Ha?. Pleurotomaria Anglic a and cast Lias. last-mentioned figure (b, fig. 59), that an empty space shaded dark, which the shell itself once occupied, now intervenes between the enveloping stone and the cast of the smooth interior of the whorls. In such cases the shell has been dissolved and the component particles removed by water percolating the rock. If the nucleus were taken out a hollow mould would remain,
. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. Phasianella Heddingtonensis, and cast of the same. Coral Ha?. Pleurotomaria Anglic a and cast Lias. last-mentioned figure (b, fig. 59), that an empty space shaded dark, which the shell itself once occupied, now intervenes between the enveloping stone and the cast of the smooth interior of the whorls. In such cases the shell has been dissolved and the component particles removed by water percolating the rock. If the nucleus were taken out a hollow mould would remain, on which the external form of the shell with its tubercles and striae, as seen in a, fig. 59, would be seen embossed. Now if the space alluded to between the nucleus and the impression, instead of being left empty, has been filled up with calcareous spar, flint, py- rites, or other mineral, we then obtain from the mould an exact cast both of the external and internal form of the original shell. In this manner silicified casts of shells have been formed; and if the mud or sand of the nucleus happen to be incoherent, or soluble in acid, we can then pro- cure in flint an empty shell, which in shape is the exact counterpart of the original. This cast may be compared to a bronze statue, representing merely the superficial form, and not the internal organization ; but there is another description of petrifaction by no means uncommon, and of a much more wonderful kind, which may be compared to certain anatom- ical models in wax, where not only the outward forms and features, but the nerves, blood-vessels, and other internal organs are also shown. Thus we find corals, originally calcareous, in which not only the general shape, but also the minute and complicated internal organization are re- tained in flint. • Such a process of petrifaction is still more remarkably exhibited in fossil wood, in which we often perceive not only the rings of annual growth, but all the minute vessels and
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Keywords: ., bookcentury1800, bookdecade1860, booksubjectgeology, bookyear1868
