92 STRUCTURAL AND FIELD GEOLOGY 
an empty bivalve or univalve shell is enclosed in a deposit, 
the sediment usually at the same time fills the vacuity. 
Afterwards, the shell itself may be gradually dissolved and 
removed by percolating water. The cavity thus formed may 
be subsequently reoccupied by mineral matter, and in this 
way a perfect cast will be produced. Not infrequently, how- 
ever, the space left by the shell remains unfilled, containing 
in its centre the stony kernel which formerly occupied the 
interior of the original. Should this kernel not adhere to 
the matrix, it will rattle, like a nut in its shell, when the 
specimen containing the fossil is shaken. 
Permeation and Molecular Replacement. — Mineral 
matter has often thoroughly permeated an organic body, 
and filled up all its pores and cavities—a process which 
has usually been preceded, accompanied, or followed by 
carbonisation. Not infrequently, under these conditions, the 
original substance itself is more or less molecularly replaced 
by mineral matter, with partial or perfect preservation of the 
internal structure. This kind of fossilisation is well illustrated 
by some specimens of silicified wood, the minutest structures 
of which have been so completely replaced that a slice of the 
specimen, viewed under the microscope, reveals as much as 
a section of the original wood itself could have shown, 
Permeation and molecular replacement may be exemplified 
by one and the same fossil, so that the two kinds of 
fossilisation are frequently hard to distinguish. An organic 
body which is permeated and molecularly replaced by mineral 
matter is a true petrifaction. 
In cases of true petrifaction, the replacing mineral is 
usually either silica (mainly chalcedony or opal) or calcium 
carbonate. The same substances also play the most 
important part in the formation of incrustations and casts, 
which is just what might have been expected when we 
remember how very widely calcareous and siliceous solutions 
are diffused. Other substances, however, not infrequently 
replace organic remains, such as the compounds of iron 
(pyrite, marcasite, hematite, limonite, and siderite), and, less 
frequently, gypsum, barytes, fluor-spar, and various metals 
and metallic compounds. 
It is not only the relics and remains of plants and animals 


