^^'o.l.] WHITE ON CONDITIONS OF PRESERVATION OF FOSSILS. 135 



wood tlian iu any other, where not only the character and appearance 

 of the woody fibre is perfectly retained, but the microscopic structure 

 is often so perfectly preserved as to distinguish the botanical divi- 

 sion to which it belongs. In the case of pseudomorphic silicification 

 of invertebrate fossils, the microscopic structure is seldom, if ever, so 

 perfectly preserved as it is in the case of wood, but their true pseudo- 

 morphism, nevertheless, occurs, the more common cases of which consist 

 of the silicification of shells, corals, &c., in limestone. In these cases, 

 the objects were without doubt first fossilized as calcareous substances, 

 and as such they could not have been removed by solution, leaving their 

 moulds in the matrix, because that matrix was of essentially the same 

 composition as themselves and nearly or quite equally soluble. There- 

 fore it is evident that the change of substance must have been effected 

 by the process of pseudomorphism which has just been explained, and 

 which was initiated by a movement and segregation of the sihceous 

 atoms that were previously distributed in the substance of the imbed- 

 4ling rock, which movement and segregation were much the same as 

 took place in the formation of flint-nodules in chalk and similar sili- 

 ceous nodules and masses that are often met with in limestones. That 

 this pseudomorphism of calcareous fossils by silicification began long 

 after they had become perfectly fossilized, is indicated by the fact that 

 the process has, in most cases at least, taken place in connection with 

 the weathering of the strata which contain the fossils. For example, it 

 is very common to find shells, corals, crinoid fragments, &c., wholly or 

 partly silicified upon and near the surfaces of certain paleozoic limestone 

 strata, as well as in the interstices which are filled with the debris of 

 their decomj)osition ; while fossils of the same kinds which are imbedded 

 in the solid and un weathered iiortions of the same strata, are wholly 

 calcareous. Although fossilization proper of calcareous remains is more 

 complete in limestones than in other rocks, it is in limestones also that 

 most of the cases of silicious i)seudomorphism of invertebrate remains, 

 as just defined, takes place, the latter cases as compared with the former 

 being very few. 



Casts and moulds are more common in sandstones than in other rocks, 

 although they are not uncommon in magnesian limestones, and in both 

 sandy and argillaceous shales. The common occurrence of moulds in 

 sandstone is easily accounted for by the porous character of the rock, 

 which admits of tlie percolation of water charged with solvents of lime- 

 carbonate. Such percolation is, of course, necessary in the production 

 of moulds in all kinds of rock, but it is more complete in sandstones than 

 iu any other rocks. Some of the finest moulds of both fresh- water and 

 marine species, those which preserve not only the form, but also the 

 delicate surface-markings of the fossils most perfectly, are found in 

 hematite ; but such instances are very rare compared with the occur- 

 rence of moulds in sandstones. While ordinary limestones are, as a rule, 

 more fossiliferous than any other rocks, magnesian limestones notably 



