Chap. XL] DEVONIAN ROCKS IN DEVON AND CORNWALL. 
271 
the Caithness flags, other fossil Plants of large size begin to appear ; and 
several of these have been discovered in the Orkney and Shetland Islands 
by Dr. Hamilton, which, belonging either to Calamites * or an allied genus 
(Bornia), make a near approach to forms usually considered characteristic 
of the Carboniferous era. 
Devonian Bocks (the equivalents of the Old Red Sandstone) in Devon and 
Cornwall. — The crystalline and slaty condition of many of the stratified 
deposits in North and South Devon and Cornwall, and their association 
with eruptive rocks and much metalliferous matter, might well induce the 
earlier geologists to class them among the very oldest deposits of the British 
Isles. In truth, the south-western extremity of England presented 
apparently no regular sedimentary succession by which its grey, slaty 
schists, marbles, and siliceous grits and sandstones could be connected 
with any one of the British deposits the age of which was well as- 
certained. The establishment of the Silurian system, and the proofs it 
afforded of the entire separation of its fossils from those of the Carboniferous 
era, was the first step in the inquiries which led to a right understanding 
of the age of these deposits. The next was the proof obtained by Professor 
Sedgwick and myself, that the * culm- measures ' of Devon are truly of the 
age of the Lower Carboniferous period, and that they graduate downwards 
into some of the slaty rocks of this region. Hence it became manifest 
that the rocks now under consideration were the immediate precursors 
of the Coal-deposits, and stand, therefore, clearly in the place of the 
Old Eed Sandstone of other regions. The highly important deduction 
of Mr. Lonsdale, also, that the fossils of the South-Devon limestones, 
collected by Mr. Austen and others, really constituted a natural-history 
group intermediate between those of the Silurian rocks and of the 
Carboniferous Limestone, was the reason which had most weight with 
Professor Sedgwick and myself (after correlating North and South Devon) 
in inducing us to propose the term < Devonian ' f. The stratified rocks of 
Devonshire and Cornwall, highly varied in composition, and the equiva- 
lents of the Old Eed Sandstone in the regions alluded to, have also been 
illustrated by the researches of Sir Henry De la Beche, Mr. R. Godwin- 
Austen, Professor Phillips, and other geologists J. 
The most instructive of the sections published by my colleague and 
myself to illustrate the general structure of Devonshire is that of which 
the diagram at p. 272 is a reduction §. It passes across North Devon, 
from the Foreland on the Bristol Channel, to the granitic ridge of Dartmoor 
on the south, and exhibits a full succession of the Lower, Middle, and 
* Dr. Hooker, Quart. G-eol. Journ. vol. ix. states the part which he took in this classiflca- 
p. 49 ; see also vol. xiv. p. 73. tion. 
t See Keport of the British Association for the J See Keport on the Geology of Cornwall, De- 
Advancement of Science, 1836; Sedgwick and Mur- von, and West Somerset, by De la Beche, 1839, 
chison, Trans. Geol. Soc, 2nd ser., vol. v. p. 633, and on the Palaeozoic fossils of the same region, 
and Phil. Mag. vol. xi. p. 311. See the memoir of by Prof. Phillips, 1841. 
Lonsdale, Trans. Geol. Soc, 2nd ser., vol. v. p. 721, $ See Trans. Geol. Soc, 2nd ser., vol. v. pi. 50. 
in which our valued friend clearly and modestly figs. 1 & 2. 
