Ch. IV.] 



ORG-AJSTIC EEMAINS. 



39 



we have imagined at first sight that the shell a and the cast 6, fig. 59, 

 were different parts of the same fossil. The reader will observe, in the 



Fig. 68. 



Fig. 59. 



Phatiianella Heddingtonensis, 

 and cast of the same. Coral Kag 



Pleurotomaria Anglioa and 

 cast Lias. 



last-mentioned figure (6, 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 lings of annual 

 growth, but all the minute vessels and medullary rays. Many of the 

 minute cells and fibres of plants, and even those spiral vessels which in 

 the living vegetable can only be discovered by the microscope, are pre- 

 served. Among many instances, I may mention a fossil tree, 72 feet in 

 length, found at Gosforth near Newcastle, in sandstone strata associated 

 with coal. By cutting a transverse slice so thin as to transmit light, 

 and magnifying it about fifty-five times, the texture seen in fig. 60 is ex- 



