Chap. II.] 
STKUCTURE OF CAMBRIAN ROCKS. 
23 
sandstone, schist, or slate, which, though occasionally not more crystalline 
than the fossiliferous beds above them, have as yet afforded only rare 
indications of former beings. 
This is the important fact to which attention is now directed ; for in 
such instances the geologist appeals to strata which have undergone little 
or no alteration. In this enormous pile or series of early subaqueous 
sediment, composed of mud, sand, or pebbles, the successive bottoms or 
shores of a former sea, all of which had been derived from preexisting 
rocks, he has been unable, after many years of research, to detect more 
than a very few traces of former creatures. But lying upon them, and 
therefore evolved after them, other strata succeed, in which clear relics 
of a primeval ocean are discernible ; whilst these again are everywhere 
succeeded by deposits containing many organic remains of a more ad- 
vanced nature. In this way, evidences have been fairly obtained to show 
that the rocks bearing the name of Laurentian and Cambrian constitute 
the sterile natural bases of the rich deposits termed Silurian. 
The hypothesis, that all the earliest sediments have been so altered as 
to have obliterated the traces of any relics of former life which may have 
been entombed in them, is therefore opposed by examples of enormously 
thick and often finely levigated deposits beneath the richly fossiliferous 
rocks, in which, if many animal remains had ever existed, traces of them 
must have been detected. 
Few words need here be said of those crystalline masses from which in 
numerous regions the materials of some of the older sediments have been 
derived. Passing over the consideration of the most ancient granites, and 
other igneous or molten rocks, of which we cannot have distinct evidences, 
it has been shown that there are also strata (the Laurentian), now in a 
crystalline state, which either lie under or have been raised from beneath 
deposits of Cambrian and Silurian age. 
So long as there was no indication of animal remains in such funda- 
mental and crystalline strata, I speculated on their having been formed 
at a period when the heat of the crust of the earth was antagonistic 
to the existence of living beings; but with the discovery, in the Lau- 
rentian gneiss, of the Foraminifer Eozoon before alluded to, the term 
* Azoic,' as applied to this the most ancient formation which has been 
brought to light, has necessarily been set aside. Still, we are not to suppose 
that the almost entire absence of organic remains in the Laurentian rocks is 
due to their crystalline character only ; for, in ascending to the next great 
system in the geological order, we have as yet been unable to detect any 
striking advance in the creatures which lived during the next very long- 
period. This evidence, though negative, is all the more important inasmuch 
as in many regions the clay-slates and sandstones of this age, which lie 
beneath all the richly fossiliferous Silurian formations, are scarcely at all 
altered or metamorphosed. Proceeding upwards in our researches, we find 
