1897.] Fossils and Fossilization. 197 
silica, such as has evidently taken place in the corals of the 
Upper Helderberg limestone. An exception occurs in the 
Brachiopoda of the very sandy loosely coherent beds of the 
Oriskany sandstone at Cumberland, Maryland. In pocket-like 
cavites at this locality in the sandstone the molluscan remains 
are found beautifully preserved in silica, doubtless owing to 
the supersaturation of infiltrating waters with colloidal silica. 
The muscular bundles, the vascular markings of the circulat- 
ing system, the punctate surface of the mantle in mollusca are, 
however, well retained in siliceous muds or sandy limestones, 
and surpass in usefulness similar indications in limestones, 
where incipient crystallization has destroyed these by knitting 
both shell and its contents into a crystalline unit. 
Fossils are frequently removed by solution in carbonated 
waters, and possibly in waters carrying organic acids, as sug- 
gested by Dr. Julien, when “in porous masses of gravel, sand 
and clay” they become subject to saturation in such menstrua. 
Hilgard has shown the protective influence of an argillaceous 
matrix, for in the Tertiary deposits of the Southern States the 
shells were only partially destroyed in the clay layers, while 
calcareous concretions, made by the liberated lime from the 
dissolved fossils, were frequently in the more porous portions 
of the deposit. In the Palezoic sandstones generally shells are 
less common than the impressions, moulds and casts, and a 
similar process of obliteration may have removed them also. 
The replacement of invertebrate fossils by mineral pseudo- 
morphs or substitution is a very interesting and important sub- 
ject, and remains yet a peculiar method in the economy of 
nature. While it is true that silicification, or the replacement 
of organic structure by silica, is the most common and the most 
satisfactory form of this change, yet a number of other mineral 
species become transferred in organic bodies to the places occu- 
pied by the molecules of organic tissues, or of the carbonate of 
lime shells. In the Lower Silurian beds of the Galena lime- 
stone, in Wisconsin, fossils assume the substance of sulphide of 
lead (Galenite); and in Cornwall fragments of antlers, con- 
taining tin oxide, are found, wherein “the original structure 
seems to be almost entirely reproduced as cassiterite ” (Phillips). 
