610 PALHONTOLOGICAL GEOLOGY. — [Boox V. | 
mere hollow mould of its form remains. Among stratified rocks these 
moulds are of frequent occurrence. They may be subsequently filled 
up by mineral matter washed in mechanically or deposited as a 
chemical precipitate. Such casts are particularly common in sand- 
stone, which, being a porous rock, has allowed water to filter through it 
and remove the substance of enclosed plant-stems, shells, &e. In 
the sandstones of the Carboniferous system casts in compacted sand 
of stems of lepidodendron and other plants are abundant. It is obvious 
that in casts of this kind no trace remains of the original structure 
of the organism, save merely of its external form. 
(3.) The original substance is molecularly replaced by mineral matter 
with partial or entire preservation of internal structwre.—This is the only 
true petrifaction. The process consists in the abstraction of the 
organic substances, molecule by molecule, and in their replacement by 
precipitated mineral matter. So gradual and thorough has this inter- 
change often been, that the minutest structures of plant and animal 
have been perfectly preserved. Silicified wood is a familiar example. 
The chief substance which has replaced organic forms in rock 
formations is calcite, either crystalline or in an amorphous granular 
condition. In assuming a crystalline (or fibrous) form this mineral has 
often observed a symmetrical grouping of its component individuals, 
these being usually placed with their long axes perpendicular to the 
surface of an organism. In many cases among invertebrate remains the 
calcite now visible is pseudomorphous after aragonite (p. 166), Next 
in abundance as a petrifying medium is silica, most commonly in the 
colloid form (calcedony, opal), but also as quartz. It is specially frequent 
in some limestones, as chert and flint, replacing the carbonate of lime 
in molluses, echinoderms, corals, &c. It also occurs in irregular aggre- 
gates in which organisms are sometimes beautifully preserved. It 
forms a frequent material for the petrifaction of fossil wood. Silicifica- 
tion, or the replacement of organisms by silica, is the process by which 
minute organic structures have been most perfectly preserved. In a 
microscopic section of silicified wood, the organization of the original 
plant may be as distinct as in the section of any modern tree. Pyrites: 
and marcasite are common replacing minerals, especially in argillaceous- 
deposits, as, for example, among the clays of Jurassic and Cretaceous 
formations. Siderite has played a similar part among the ironstones 
of the coal-measures, where shells (Anthracosia, &c.) and plants have 
been replaced by it. Many other minerals are occasionally found to 
have been substituted for the original substance of organic remains. 
Among these may be mentioned glauconite (replacing or filling fora- 
minifera), vivianite (specially frequent as a coating on the weathered 
surface of scales and bones), barytes, celestine, gypsum, tale, lead- 
sulphate, carbonate, and sulphide, copper-sulphide and native copper, 
hematite and limonite, zinc-carbonate and sulphide, cinnabar, sulphur, 
fluorite, phosphorite." 
iii. Relative Paleontological Value of Organic Remains.—As 
the conditions for the preservation of organic remains exist more | 
* Roth, Chem. Geol. i, p. 605. 
