Chap. II.]                               CAMBEIAN EOCKS.
 

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CHAPTER II.
CAMBEIAN EOCKS.

OUTLINES, STRUCTURE, AND ORDER OF THE ROCKS NEXT ABOVE THE LAURENTIAN.—THE RAKE
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 outhne in difi'erent 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 crystalhne 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 consohdated 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 crystaUine condition.     So again in the United States and British

* Russia and the Ural Mountains, by Murchison, de Verneuil, and von Keyserling, vol. i. p. 26*.