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X. On the Geology of Devon and Cornvoall, mtli reference to 
a paper read before the Geological Society on December ^th^ 
1839. By the Rev. D. Williams, F.G.S. 
To the Editors of the Philosophical Magazine and Journal. 
Gentlemen, 
A S I do not consider the substance and spirit of my paper 
on Devon and Cornwall, which was read at the meeting 
of the Geological Society on the 4th inst. is fairly reported 
in the Athenaeum of the 7th, I request you will favour me 
with an opportunity of righting myself with your readers, and 
of reporting progress since my communication which was 
published in your last October Journal (vol. xv. p. 293). I feel 
assured that 1 am not intentionally misrepresented in the Athe- 
naeum ; the abstract however imputes to me (as I hastily read 
it before I left London) that I hold mineral characters to be 
everything and organic evidences nothing, in determining the 
relative ages of strata. Now it was very distinctly read by the 
Secretary, Mr. Darwin, that I did not consider the law pro- 
posed by Dr. Smith to be of any value in classilying the 
rocks of the earth in remote localities, if it did not suppose a 
final and universal extinction of genera and species; and in 
as much as some plants and animals would probably be en- 
abled by the Creator to survive mutations which would be 
death to others, I considered that a classification of the 
older rocks should be regulated by some per-centage test, 
such as Mr. Lyell had applied to the tertiaries, rather than 
by a more restricted rule. J quote from memory, not being 
able to refer to either the Athenaeum or to my paper. I stated 
that I unequivocally believed in the extinction of genera and 
species, severally at distant epochs, and therefore did not be- 
lieve that the Posidonia and Goniatite, which I discovered in 
some trashy lentiform limestones in Devonshire, were specially 
created for the mountain limestone alone, when I knew it 
could be proved to demonstration, that those Posidonia lime- 
stones of Devon, and all their associated rocks, not only bore 
no lithological resemblance to any of the mineral types of any 
portion of the great English coal-field, but that they under- 
laid the coral limestones of South Devon, and the whole of 
the slates of Cornwall. I exhibited sections evidencing the ac- 
tual supraposition of the Cornish killas on the floriferous 
series. No, 9, and the Coddon grit, No. 8, and I pointed out 
the localities. In all fairness then it remains for gentlemen 
to disprove those facts, instead of requiring me to show what 
I believe to be an impossibility, viz. the identity of the 
plants I found in the great floriferous outlier on the south and 
