1897.] Fossils and Fossilization. 195 
The limestone formations which represent the accreted de- 
posits of foraminiferous ooze, shells, and the lime paste made 
by the solution of shells, contain fossils in great abundance, and 
yield excellent specimens. But specimens in individual per- 
fection, such as are extracted full and free from their matrix, 
are mainly obtained from impure limestone rocks. The ad- 
mixture of clay or sand differentiates as it were the matrix from 
its included fossils, and they seem less consolidated and blended 
with the surrounding rock, so that their outlines form boundar- 
ies of separation, and the fossils are picked out complete, quite 
disengaged from any adhering stone. Such beds as the Hud- 
son River Slates, the Waldron beds of Indiana, the Hamilton 
layers in New York, the Lower Carboniferous shale at Craw- 
fordsville, Indiana, are illustrations of a fossiliferous rock from 
which. the fossils become detached, retaining their surface 
characters and a clean, hardened epidermis, from which every 
particle of rock can be separated. In siliceous limestones, like 
the Schoharie Grit, the Oriskany Sandstone, and the Calcifer- 
ous beds along Lake Champlain, the fossils are revealed by 
weathering, whereby the limestone seems displaced by solution 
in surface waters, or slowly lifted from the contour of fossils by 
frost decrepitation, and the fossils remain as partial or entirely 
siliceous reliefs. Prof. Perkins, of Vermont, has found produc- 
tive quarries of fossils under growing trees, where the vegetable 
acids have disintegrated the calcareous portions of these beds, 
and left the silicified fossils in complete relief or entirely free, 
as if shaken out from the enclosing envelopes of rock. In hard 
limestones this surface weathering often takes place, a sort of 
aerial development, by which the valves of shells, the spines 
and members of trilobites, the stems and plates of crinoids, 
slowly emerge and stand out on a rock, whose interior faces 
only reveal a poorly discerned outline of fossils, until they 
have undergone this atmospheric alteration. The fossils them- 
Selves appear to have resisted the attack of acid waters and 
frost from having a greater density, upon which these agents of 
change failed to act, or from having become silicified in the 
Process of change. In many of these beds the fossils are col- 
ored by iron oxide, caused by weathering, from a protoxide to 
