THE 
AMERICAN NATURALIST 
VOL, AX XI. March, 1897. 363 
. FOSSILS AND FOSSILIZATION. 
By L. P. Gratacap. 
IV. 
(Continued from p. 33.) 
As regards their degree of preservation the nature of the 
deposit has some influence upon the condition of fossils; and 
the nature of a deposit involves also a suggestion of its position, 
as we have seen, whether subject to estuarine vicissitudes, tidal 
fluctuations, or the ceaseless attrition of shore waves. When 
we examine the sandy and gravelly deposits of the Potsdam 
(Cambrian), the Medina (Upper Silurian), the Chemung and 
the Catskill (Devonian), the coarse conglomerate of the Lower 
Carboniferous, or the calcareous grits of the Schoharie and 
Oriskany (Lower Devonian) we find a reflexion of their physi- 
cal character in the condition of the fossils preserved in them. 
The trilobite layers of Wisconsin and Minnesota in the Upper 
Cambrian (Potsdam) are crowded with the separated parts of 
these crustacea, whose delicate articulations were unable to re- 
sist the friction they were exposed to in the motions on a beach 
of siliceous sand. Neither does a sandy beach receive impress- 
ions with ease and certainty, as the indistinct outlines of the 
trilobites in these beds show. Furthermore, ina siliceous bed, 
more or less percolated through by water, the water has dis- 
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