198 The American Naturalist. [Mareh, 
In the Coal Measures the fossil mollusca (Solenomya, Macro- 
cheilus Orthoceras Bellerophon) are often found as earthy casts 
coated with a skin or shell of iron pyrite. Limonite, the 
hydrated sesquioxide of iron, reproduces the shells of Afrypa, 
Beyrichia, Dalmania, in the Clinton iron of the Upper Silurian, 
in Oneida Co., N. Y., while Malachite, Sphalerite, Sulphur, 
Barite, Celestite, Fluorite, Calamine, etc., in an imperfect way 
replace the shells or coverings of fossils, fill their interiors, and 
rarely take on the resemblance of their tissue and texture. 
The silicification of fossils in its simplest expression is the 
substitution of the mineral quartz—silica—for the molecules of 
the shell of an organism, or for those of its hard or horny ana- 
tomy. The exact steps by which this alteration is effected are 
not clearly known, and of the many hypotheses offered to ex- 
plain this phenomenon all may have some elements of truth. 
It was observed long ago by Von Buch that this silicification 
followed the organic structure. 
It is certainly true that organic matter, if of some consistency, 
possesses some power of arresting silica, fixing it, as it were, by 
removing it from aqueous solutions. Le Conte suggests that 
at least in the case of wood an alkaline silicate in water is neu- 
tralized by the humic acid of the decomposing vegetation, by 
which the silica of the salt is precipitated in the pores of the 
wood, and the wood fibre, as humic acid, is removed as the 
silica assumes its position. This seems to imply too much in- 
cipient decomposition to be reconciled with the very perfect 
microscopic manner in which wood texture is replaced by an 
agate pseudomorph. Dr. Julien has laid great stress upon the 
important silicic compounds of vegetable or organic acids in 
producing colloidal silica replacements. This view seems to 
be that “ during the decomposition of the sarcode of both ani- 
mal and vegetable organisms, after death, gelatinous or colloid 
substances are generated, resembling glairine, which are solu- 
ble in sea water, which combine with silica, and may therefore 
convey and concentrate it, dissolving its particles disseminated 
through submarine sediments, and which may, in forms pre 
duced by gradual oxidation, act also as acid solvents of lime, 
oxides of iron and manganese.” A similar explanation, though 
