404 
FOSSILS. 
cerued, in a state of helpless uncertainty as to the character of the 
more i)rimitive life forms. Most strata also are of marine origin, and 
fossilized land and freshwater .species are naturally seldom met with. 
Fossil, though meaning anything dug out of the ground, is re- 
stricted to the relics of organized life entombed in the rocks ; the term 
Subfossil being usually applied to them when found in beds at present 
in course of deposition. Conchologically a fossil may be the actual 
shell itself not noticeably altered, or the shell substance may have 
peri.shed, owing to the percolating water having dissolved and carried 
away in .solution the carbonate of lime of which it is composed, and 
which as dissolved may be replaced by a deposit of silica or other 
substance, sometimes preserving not only the external form and 
appearance, but even in great part the internal structure, or the cast 
or impression only, may be left, which, however, may exhibit every 
peculiarity or detail of the original shell with minute accuracy. 
All rocks containing fossils are stratified and, except those of cal- 
careous composition, are mainly formed by at|ueou.sly deposited layers 
of mud, sand, or other material derived from the waste of the land, 
and although the first or earliest stratified deposits were neces- 
sarily formed from the disintegration of the igneous rocks, yet 
sub.se(pient beds are formed in great part fi'om the waste of previou.sly 
formed strata which have become elevated above the water level, and 
in this way the same material has been worn away by atmospheric 
forces and redeposited elsewhere, and as this may have occurred again 
and again the destruction and loss of their fossil contents is almost 
assured, for as soon as dry land is formed it is immediately acted upon 
by rain, frost and other atmospheric agents, and the consequent 
erosion and denudation provides the sediment for the formation of the 
new rocks, which are deposited in layers or strata, a succession of which 
more or less similar in character is called a Formation. 
The formations are grouped together in three great divisions, known 
as the Primary, the Secondary and the Tertiary periods or Systems. 
The P vL.EOZOIC (TrdXaids', old ; life) or Primary system embraces 
the Archtean, Cambrian, Silurian, Devonian, and Carboniferous .series 
of rocks, and is remarkable for the marvellous development of the 
Trilobites, an extinct group of Crustacea, restricted to the Palaeozoic 
era, which attained its maximum about the close of the Cambrian 
period. It is also characterized by the presence of holostomatous 
Castropods, although the Pelecypods preponderate in numbers. 
