288 The American Naturalist. [April, 
(Beckite markings) is customarily assigned to the later periods 
of the fossils change, whereas the intimate replacement of the 
microscopic structure took place in the earliest stage of its 
inhumation. 
In the Trenton limestone of Wisconsin, a more or less mag- 
nesian rock, holding from one to three per cent. of soluble 
silica, the molluscan forms are replaced by silica, forming hard 
brittle pseudomorphs of much beauty ; andin the Trenton beds 
of Tennessee we find the corals frequently or universally sili- 
cified by secondary silicification (Columnaria, Tetradium, etc.). 
It would seem that the silicification of fossils, where it is of a 
minute and very accurate character, must have been begun 
before the consolidation of the limestone itself, and have been 
completed before the layers assumed their final lithological 
state. But some peculiar instances of an apparent progressive 
silicification continuous with the weathering of the enveloping 
rock, are known as where, in the Niagara limestone in wes- 
tern New York, fossils appear in relief above the surface of the 
dissolved limestone, and are seen to be complete siliceous re- 
placements, the parts of the same fossils, or other similar fos- 
sils, enclosed within the rock a short distance below its surface, 
being entirely dissolved when placed in acid, evincing their 
calcareous nature. If this is true, the replacement must take 
place from the soluble siliceous constituents of the rock, which 
are slowly introduced into the calcareous tests and frame-work 
of fossils as these are dissolved in carborated waters. It would 
seem more likely, in most cases, as in the very siliceous and 
ferruginous Schoharie grit,’ that silicification has been already 
partially effected, and that the action of natural solvents is to 
remove the associated calcareous particles and leave the sili- 
ceous residue as the representative of the fossil, somewhat less 
1In this formation the shells of fossil bivalves are often removed by solution, 
being almost entirely carbonate of lime; and the siliceous filling, colored brown 
by the ferruginous oxydation, remains as casts of their interiors. But in other 
cases silicification has partially replaced the shells, and fragments of these taken 
from unweathered portions of the rock show upon solution in acid siliceous 
scales, which remain undissolved. In the Trenton of Teneessee the weathered 
siliceous shell of gasteropods is continuous with the siliceous parts yet plainly seen 
imbedded in the unweathered limestone. 
