Chap. II.] 
CAMBEIAN EOCKS. 
21 
CHAPTER II. 
CAMBRIAN EOCKS. 
OUTLINES, STRUCTURE, AND ORDER OF THE ROCKS NEXT ABOVE THE LAURENTIAN. — THE RARE 
FOSSILS OF THE CAMBRIAN ROCKS. — THE ORDER OF CONFORMABLE SUCCESSION UPWARD TO 
THE ' PRIMORDIAL,' OR LOWEST SILURIAN, ZONE. — SLATY CLEAVAGE. — METAMORPHOSED 
CAMBRIAN ROCKS OF ANGLESEA. 
With the exception of the fundamental deposits, termed < Laurentian/ 
which, wherever they have, been observed, are highly crystalline, the other 
and overlying ancient rocks in which fossil animals have been detected 
vary much in structure and outline in different regions. Wherever such 
masses have remained in a state of comparative quiescence from the 
period when they were raised up from beneath a primeval ocean, and have 
since been modified to a slight extent only, they are necessarily unlike 
those strata which, though formed at the same period and even composed 
of similar materials, have been penetrated by igneous matter, or subjected 
to alteration through the action of mechanical pressure, heat, and other 
agencies. 
In Russia *, where some of the older deposits have been but partially 
hardened since they were accumulated at the bottom of the sea, and have 
been elevated into merely low plateaux that have undergone no great 
change or disruption, they have but little resemblance to many rocks of 
the same age in other countries. When, however, we follow these soft 
primeval Russian strata to the Ural chain, a region abounding in eruptive 
and contorted rocks, we find that the beds which on the west consist of 
mud and sand have there been converted into crystalline schists, limestone 
in the state of marble, and auriferous quartzites. In Britain, no more 
striking examples of these changes of physical conditions can be seen 
than in the territory whose geological structure is explained in this work 
and the accompanying map. Throughout that portion of the region 
(Shropshire and Herefordshire) which afforded the types for the Silurian 
classification, the strata consist of shale, sandstone, and limestone, which, 
though much more consolidated than their equivalents in European Russia, 
have been wholly unaffected by that influence which has impressed upon 
the very same beds, when followed into Wales, a true slaty cleavage. Or 
if we trace those slates of North Wales across the Menai Straits, we find 
that they have undergone another change, and have been metamorphosed 
into a crystalline condition. So again in the United States and British 
* Kussia and the Ural Mountains, by Murchison, de Verneuil, and yon Keyserling, vol. i. p. 26*. 
