56 



LYELL'S ELEMENTS OF GEOLOGY. 



Mineralization of Organic Remains. 



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 strise, as seen in a, Fig. 

 55., 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, pyrites, 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 procure in flint an empty shell which 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 petri- 

 faction by no means uncommon, and of a much more wonder- 

 ful kind, which may be compared to certain anatomical 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 retained in flint. 



Such a process of petrifaction is still more remarkably exhi- 

 bited in fossil wood, in which we often perceive not only the 

 rings of annual growth, but all the minute vessels and medullary 

 rays. Many of the minute pores and fibres of plants, and even 

 those spiral vessels which in the living vegetable can only be 

 discovered by the microscope, are preserved. Among many 

 instances I may mention a fossil tree, seventy-two feet in length, 

 found at Gosforth near Newcastle, in sandstone strata associated 

 with coal. But cutting a transverse slice so thin as to transmit 

 light, and magnifying it about fifty-five times, the texture seen 

 in Fig. 56. is exhibited. A texture equally minute and compli- 

 cated has been observed in the wood of large trunks of fossil 

 ■p-^ trees found in the Craigleith quarry near 



Edinburgh, where the stone was not in 

 the slightest degree sihceous, but consisted 

 chiefly of carbonate of lime, with oxide 

 of iron, alumina, and carbon. In some 

 examples the woody fibre is partially pre- 

 served, but it has entirely vanished from 

 others. 



In attempting to explain the process of 



Texture of a tree from the -•/•^• • i 



coal-strata, magnified. (Wi- petrifaction m such cascs, we may first 

 assume that strata are very generally per- 



