Chap. VIII.l             SILUEIAN EOCKS OF COENWALL.                            145
 

CHAPTER VIII.
SILUEIAN EOCKS OF BEITAIN

beyond the original typical region—NAMELY, IN CORNWALL, THE NORTH-WEST OF
ENGLAND, SCOTLAND, AND IRELAND.

Though occupying large spaces in other parts of the British Isles, the SUu¬
rian rocks, as separated into formations and characterized by fossUs, are
nowhere so clearly defined as in the typical portion of the region of which
we have taken leave. In no other tract of the United Kingdom have geolo¬
gists been able to show so clearly the relations of the different members of
the lower fossUiferous rocks, nor so clear an ascending order from the SUurian
into the next overlying deposit, the Old Eed Sandstone or Devonian rocks.

In CornwaU, for example, the discovery of certain fossils has proved that
some of the quartzose sandstones forming its southern headlands are Lower
Silurian. The fossiliferous sandstone in question, passing to the south of
the Hodman, and coming out to view in Gorran Haven, contains several
species of Orthidae, as weU as TrUobites, which characterize that division *.

There, however, no one can show an unbroken descending sequence be¬
neath those beds, nor an ascending order from them to the younger deposits
of Upper SUurian and Devonian age; and as large portions of the strata of
CornwaU have been highly altered and mineralized, so also is this southern
tract much dislocated. In such a region, therefore, we cannot expect to
meet with proofs of succession. It is sufficient to state that the band of
grits and quartzites in the south of CornwaU, which I termed SUurian
in 1846, presents much of the character and aspect of the opposite rocks of
Cherbourg and Brittany, which the French geologists have mapped and de¬
scribed as Lower Silurian (see Chapter XVII.). This is precisely one of those
broken and insulated tracts of older sedimentary rocks where the geologist
has no other test by which he can recognize age than their imbedded
organic remains.

Professor Sedgwick, who visited the localities after I had described
them, shows, indeed, that these strata are inverted, and overhe the De¬
vonian or Old Eed rocks. The chief fossUs defined by Sedgwick and
M'Coy t consist of the simple-plaited Orthidae so common in the Lower
Silurian rocks (Pis. V. & VI.), viz.: Orthis calhgramma, 0. fiabeUulum, 0.
elegantula, and 0. testudinaria; Strophomena grandis; and the TrUobites

* These fossils were collected by Mr. Peach;     G-eological Map of Enrfand and Wales, published
and, from their inspection and a visit to their     by the Society for the Diffusion of Useful Know-
chief localities in 1846, I termed the rocks in     ledge.

which they occur Lower Silurian (Trans. Eoy.        t See Quart. Journ. Greol. Soc. Lond. vol. viii.

G-eol. Soc. Cornwall, 1846, p. 317); and as such     p. 13.
they were inserted in a new edition of my little