Chap. VIII.] 
SILUKIAN ROCKS OF CORNWALL. 
145 
CHAPTER VIII. 
SILUKIAN ROCKS OF BRITAIN 
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 Silu- 
rian rocks, as separated into formations and characterized by fossils, are • 
nowhere so clearly denned 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 fossiliferous rocks, nor so clear an ascending order from the Silurian 
into the next overlying deposit, the Old Red Sandstone or Devonian rocks. 
In Cornwall, 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 Dodman, and coming out to view in Gorran Haven, contains several 
species of Orthidse, as well as Trilobites, 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 Silurian and Devonian age ; and as large portions of the strata of 
Cornwall 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 Cornwall, which I termed Silurian 
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 overlie the De- 
vonian or Old Eed rocks. The chief fossils denned by Sedgwick and 
M'Coyf consist of the simple -plaited Orthidse so common in the Lower 
Silurian rocks (Pis. V. & VI.), viz. : Orthis calligramma, 0. flabellulum, 0. 
elegantula, and 0. testudinaria ; Strophomena grandis ; and the Trilobites 
* These fossils were collected by Mr. Peach; Geological Map of England 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. Geol. 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 
L 
