> 
1848. ] SEDGWICK ON THE SKIDDAW SLATE. 219 
the groups above described, first taking the rocks of the Silurian 
system as our type. The lower part only of the group (No. 4), 
Coniston limestone and flagstone, contains true Lower Silurian species. 
All the higher parts of the section are, therefore, Upper Silurian till 
we touch on the red conglomerates. A re-arrangement on fossil 
evidence only disturbs one great physical group (No. 4) ; the 
others fall into strict zoological coordination. In lke manner the 
results derived from the Westmoreland section only disturb one of 
Sir R. I. Murchison’s groups ; by teaching us (what indeed would 
follow from fossil evidence) that the tilestone is to be regarded, not 
as a part of the old red sandstone, but as the top of the Upper 
Ludlow series. Surely if a conclusion like this proves the great 
value of fossil evidence, it also shows the great importance of defi- 
nite physical groups of deposits; one set of phenomena running 
in very near coordination with the other*. In further illustration of 
what is here stated, I may appeal to the recent labours of Mr. Prest- 
wich in determining the true comparison between the London and 
Hampshire tertiary basins. It is not by accumulating descriptions of 
organic remains, or by elaborate sections, that he has worked out his 
evidence ; but it is by taking and weighing together both species of 
evidence that he has produced a beautiful coordination between the 
distant parts of our contemporaneous tertiary deposits. 
Taking for granted what has been above stated, viz. that the 
Coniston limestone and a part of the overlying flagstone represent 
the Lower Silurian rocks, and that a portion of (No. 4) and all the 
other overlying groups do in order represent on a noble scale the 
whole Upper Silurian system,—what are we to say of the green 
slates and porphyries ? and what of the Skiddaw slate ? 
I have before stated my conviction that the great group (No. 3) 
was the true equivalent of the great Cambrian group (or groups) of 
North Wales, forming the higher mountains of Carnarvonshire and 
Merionethshire. But the great Welsh group contains fossils almost 
to its base, though in the lower part of it they become very rare. 
The great Cumberland group has not yet been proved to contain a 
single fossil. This I have endeavoured to explain by referring the 
negative fact to the enormous masses of contemporaneous igneous 
products (either direct or recomposed) which have either destroyed 
the traces of organic life, or prevented the development of organized 
bodies during the long period of the older Cumbrian slates. Here, 
then, was an imperfection in the generalization. It was formed 
partly on hypothesis, and partly on physical development and geo- 
logical position ; but not on positive identity of fossils. But the 
Skiddaw slate is not metamorphic, (except in its lower part, which is 
highly metamorphic, and which I do not profess to describe in this 
paper,) and contains many arenaceous and earthy beds, in which we 
might expect to find traces of fossils, had such existed during the 
period of deposit. I have stated in former papers, that I found no 
* Tf our classification had been based on the Westmoreland sections, I think 
No. 5 would have been regarded as the commencement of the Upper Silurian 
series. 
