700 Professor Sedgwick and R. I. Murchison, Esq., on the 



many of the best-known mountain-limestone fossils from the carbonaceous 

 shales and sandstones of the lower culm measures of Devonshire. 



2. Again, it has been objected, that the fossils of the Devonian groups, now 

 classed with the old red sandstone, are not the same with the known fossils of 

 that formation. 



We allow that the fossil fish of the old red sandstone have not yet been dis- 

 covered among the Devonian rocks. But how can we build any conclusion 

 on such a mere negative fact } The same genera and species are not distri- 

 buted through the whole extent of every formation. Moreover, who can tell 

 that fossil fish will not hereafter be discovered among the calcareous slates of 

 Devonshire.? But there is a more positive objection to be noticed, viz. that 

 the known fossil shells of the old red sandstone differ greatly from those we 

 have obtained from the Devonian groups. To this we may reply, that the 

 shells hitherto described from the old red sandstone are derived exclusively 

 from its very lowest beds, which probably do not appear on any of our pre- 

 vious sections. These objections are, however, of little weight against the 

 broad fact (which, in truth, forms the sum and substance of our evidence), 

 that the fossils described in this paper are of a type intermediate between 

 the types of the carboniferous and Silurian systems*. 



3. The extreme change, among the formations we have described, of the 

 ordinary mineral type, is another objection to our views. It has been noticed 

 in various parts of our two memoirs, and considered at some length in our 

 paper, of April last, in the Philosophical Magazine ; we need not, therefore, 

 dwell upon it now. But we may briefly state, that as the old red sandstone 

 exhibits so many changes of structure, in its range from the Orkneys to the 

 north side of the Bristol Channel, we have no difficulty in admitting still fur- 



* Were it true, that species abounding in the mountain limestone, and disappearing in the de- 

 scending sections, appeared again in still lower parts of the gray-vvacke series ; there would then 

 be comparatively little weight in our evidence, or certainty in our conclusions. But we have no 

 such antagonist fact now to contend with. In a paper, published (April 1839) in the Philoso- 

 phical Magazine, we expressed our hopes that the views of classification we then threw out might 

 cast some light on the structure of the south of Ireland ; and before our paper was struck off, we 

 became acquainted with the contents of a memoir by Mr. C. W. Hamilton, to which we alluded 

 in a postscript. At the very meeting which followed that on which our present paper was read 

 before the Geological Society, Mr. Griffith exhibited a copy of his Geological Map of Ireland, 

 and read a memoir, illustrated by sections, and accompanied by fossils ; by help of which he 

 showed, we think, to demonstration, that there is no anomalous recurrence of the carboniferous 

 fossils among the true gray-wacke rocks of the south of Ireland. His sections of the rocks im- 

 mediately below the mountain limestone exhibited a broken, imperfect series ; most instructive, 

 however, in one respect, as showing an undoubted ascending passage into the limestone. 



