1897.] Fossils and Fossilization. 289 
dense in texture, but a substantial and complete form. In fact, 
in the case of many specimens of weathered fossils, where the 
siliceous shells project in high relief above the limestone matrix, 
the surfaces of the shell are finely perforate, or small holes 
occur, as if the limestone had been removed from these spaces 
and the intervening areas of silica remained imperfectly con- 
tinuous. Silicification seems to have gone on with great energy 
and completeness in some beds in the same formation in which 
other beds show but imperfect traces of its action, and this may 
be ascribed to a greater proportion of soluble silica, and per- 
haps as well to the presence of organic structures more sus- 
ceptible to siliceous replacement. As regards the first course, 
it is true that mere excess of a quartzose matrix does not nec- 
essarily facilitate silicification, as we see in the Oriskany sand- 
stone, which is so frequently characterized by cavities from 
Which fossils have been dissolved by carbonated waters, though 
each one of those cavities is surrounded by sandstone. The 
extraction of this silica could not in this case be effected so as 
to replace the calcareous parts of the dissolved fossils, because 
of its insoluble nature. Soluble silica, that colloidal form 
which is more readily taken in solution, must be provided, for 
the substitution of the lime portions of fossils. As regards the 
second cause, it seems certain that thin and delicate tests or 
structures, as the septa and tabule of corals, the partitions and 
walls of bryozoans, and the fibrous texture of some brachiopo- 
dous shells are more susceptible to replacement by dissolved 
Silica than the valves of lamellibranchs or the whorls of gas- 
teropods. 
Sorby has called attention to an interesting siliceous re- 
Placement in the calcareous grit below the coralline oolite in 
England, where very small reniform bodies occur, converted 
Into agate or presenting microscopic geodes, whose interior 
walls were lined with an agate film. Sometimes these reniform 
bodies are filled with calcareous spar, and these contrasted 
fillings are seen in the same slide side by side. Whatever these 
enigmatical bodies really are (Sorby was inclined to regard 
them as foraminiferous), they illustrate the minute way in 
Which silicification acts, for they are on an average about zbo 
