C.—GEOLOGY. 58 
lie in isolated basins and are variable in composition ; hence, they could 
not be correlated either by continuity in the field or by their lithology. 
Lyell classified them by the percentage of their species of fossils that are 
still living ; and this method, supplemented later by the use of type fossils, 
secured the world-wide recognition of his four Kainozoic Systems. 
Lyell’s achievement was followed by the foundation of the Silurian 
System by Murchison. In 1831 all the rocks below the Carboniferous 
were included in the Primary Division, of which the upper part was known 
as the ‘ Grauwacke Group.’ Murchison, in 1834, showed that the ‘ Upper 
Grauwacke Series’ included four fossiliferous series—the Builth and 
Llandeilo Flags, the Mayhill, the Wenlock and Dudley, and the Ludlow. 
In July, 1835,? he introduced the name Silurian to cover these four series, 
which he then renamed the Llandeilo, Caradoc, Wenlock, and Ludlow. 
The underlying beds were left as the ‘Slaty Grauwacke.’ Sedgwick,® a 
month later, in a communication to the Association at its Dublin Meeting, 
founded the Cambrian System for the fossiliferous rocks below the 
Llandeilo and the schists of Anglesey and Carnarvonshire. 
The fundamental advance in Geology in the decade beginning 1830 
was Lyell’s demonstration of the uniformity of geological dynamics. The 
first volume of his Principles was published in 1830, and Murchison, in 
1832, hailed it in his Presidential Address to the Geological Society as 
‘beginning to unfold the true papyri of geological history.’ Conybeare, 
in his Report to the Association, said it marked ‘ almost a new era in the 
history of our science.’ 
A very different estimate was expressed by Adam Sedgwick, then the 
leader of British Geology, in his Presidential Address to the Geological 
Society in 1831 ; he declared that Lyell’s championship of uniformitarian- 
ism violated sound reasoning on geological phenomena, and that ‘ warped 
by his hypothesis . . . in the language of an advocate, he sometimes 
forgets the character of an historian.’ According to Sedgwick, if Lyell’s 
views of the uniform order of physical events were correct, ‘ the earth’s 
surface ought to present an indefinite succession of similar phenomena. 
But as far as I have consulted the book of nature, I would invert the 
negative in this proposition, and affirm that the earth’s surface presents 
a definite succession of dissimilar phenomena. If this be true, and we 
are all agreed that it is; and if it be also true, that we know nothing of 
secondary causes, but by the effects they have produced, then “ the un- 
deviating uniformity of secondary causes,” “the uniform order of physical 
events,” ‘the invariable constancy in the order of nature,” and other 
phrases of like kind, are to me, as far as regards the phenomena of geology, 
words almost without meaning. They may serve to enunciate the pro- 
positions of an hypothesis; but they do not describe the true order of 
nature.’ 
Sedgwick agreed with Brongniart that the Geological and Historical 
Periods were essentially distinct ; and he remarked regarding the recent 
appearance of man, ‘ were there no other zoological fact in secondary 
2 © On the Silurian System of Rocks.’ Phil. Mag., vol. VII, July, 1835, pp. 46-52. 
8 5th Rep. B.A. for 1835-1836, Comm., pp. 60-61. 
4 Proc. Geol. Soc., vol. I, 1831, pp. 303, 304-5. 
