Paleozoic Rocks of the British Isles. 515 
configuration of that remarkable region, and of the changes it has 
undergone. In regard to the older Paleozoic rocks, referring to a 
memoir which he read before this Society in 1832, he still adheres 
to the threefold division of the slate-rocks of Cumberland proposed 
by Mr. Jonathan Otley. With the two lowest of these, the Skiddaw 
slate and the green slate and its associated porphyry, we need not 
now concern ourselves, for they contain no organic remains. The 
third division, or the Upper Slaty rocks, is considered by Professor 
Sedgwick to represent the Silurian series. He separates it into three 
groups, the uppermost of which he compares with the Ludlow rocks, 
the second is an ill-defined hard siliceous mass, with no good fossils, 
and the lowest consisting of the Ireleth slate and limestone, inclu- 
ding the Coniston, is proved by its fossils to be of the age of the Lower 
Silurian rocks. This view is, in short, that which has been for some 
time entertained by‘Professor Sedgwick*, and is essentially the same 
as that taken by Mr. James Marshall. 
The Jabours of Mr. Daniel Sharpe, who had commenced during the pre- 
ceding year a more detailed inquiry into the subdivisions of this series, are 
next adverted to and discussed; and after noticing Mr. Sharpe’s section 
from the head of the lake of Bala to Dinas Mowddy and Mallwyd, and his 
discovery, in the Lower Silurian rocks of the Bala territory, of the Il/lenus 
crassicauda, a trilobite eminently characteristic of the inferior strata of the 
same age in Scandinayia and Russia, the President proceeds, with reference 
to this Lower Silurian group,— 
The beds of this group are stated to rest against an unconform- 
able mass of clay-slate forming a portion of the Berwyn chain; and 
to this rock, which is void of fossils, and all that lie beneath it, our 
author would restrain the application of the word “ Cambrian.” 
This reasoning is clear, but the author must excuse me if I re- 
mind him that no definite base-line of the Palaeozoic rocks can be 
established by one transverse section only, which terminates in 
the centre of a very complicated region. He must know, that 
deposits which have no existence in a given territory set on and 
expand in adjacent tracts. Although, then, the structure of the 
Lake Country naturally gives him confidence in defining the base 
of the Silurian system by the comparison of the Bala rocks with 
those of Coniston, still it remains to be proved, whether the 
north-western tracts of North Wales do not contain other fossili- 
ferous bands inferior to those of Balas As Mr. Sharpe has not 
examined this part of the country, the question must be answered 
by others; and I rejoice to say that the reply is about to be made 
by the geologist, who, above all others, is most conversant with that 
region. 
es are aware, Gentlemen, that this is the very tract in which 
Professor Sedgwick has so long worked, and from surveys of 
which he gained that intimate knowledge of slaty structure, which 
is now considered, thanks to his masterly memoir, an essential ele- 
ment in practical geology. I dwell upon this point with peculiar 
pleasure, because I well recollect the day when the truth of those 
* See Proceedings Geol, Society, vol. iii. p. 545, et seq. 
