SUPPOSED AZOIC PERIOD. 
603 
lies of rocks, and not as definite mineral compounds. They 
are more uniform in aspect than sedimentary strata, because 
these last are often composed of fragments varying greatly 
in form, size, and color, and contain fossils of different shapes 
and mineral composition, and acquire a variety of tints from 
the mixture of various kinds of sediment. The materials of 
such strata, if they underwent metamorphism, would be sub¬ 
ject to chemical laws, simple and uniform in their action, the 
same in every climate, and wholly undisturbed by mechanic¬ 
al and organic causes. It would, however, be a great error 
to assume, as some have done, that the hypogene rocks, con¬ 
sidered as aggregates of simple minerals, are really more 
homogeneous in their composition than the several members 
of the sedimentary series. Not only do the proportional 
quantities of feldspar, quartz, mica, hornblende, and other 
minerals, vary in hypogene rocks bearing the same name; 
but what is still more important, the ingredients, as we have 
seen, of the same simple mineral are not always constant (see 
p, 503, and Table, p. 499). 
Supposed Azoic Period.—The total absence of any trace of 
fossils has inclined many geologists to attribute the origin 
of the most ancient strata to an azoic period, or one ante¬ 
cedent to the existence of organic beings. Admitting, they 
say, the obliteration, in some cases, of fossils by plutonic ac¬ 
tion, we might still expect that traces of them would oftener 
be found in certain ancient systems of slate which can scarce¬ 
ly be said to have assumed a crystalline structure. But in 
urging this argument it seems to have been forgotten that 
there are stratified formations of enormous thickness, and of 
various ages, some of them even of Tertiary date, and which 
we know were formed after the earth had become the abode 
of living creatures, which are, nevertheless, in some districts, 
entirely destitute of all vestiges of organic bodies. In some, 
the traces of fossils may have been effaced by water and 
acids, at many successive periods; indeed the removal of 
the calcareous matter of fossil shells is proved by the fact of 
such organic remains being often replaced by silex or other 
minerals, and sometimes by the space once occupied by the 
fossil being left empty, or only marked by a faint impression. 
Those who believed the hypogene rocks to have origina¬ 
ted antecedently to the creation of organic beings, imputed 
the absence of lime, so remarkable in metamorphic strata, to 
the non-existence of those mollusca and zoophytes by which 
shells and corals are secreted ; but when we ascribe the crys¬ 
talline formations to plutonic action, it is natural to inquire 
whether this action itself may not tend to expel carbonic 
