Geological Society. 147 



" Observations on the occurrence of Freshwater Beds in the Ooli- 

 tic Deposits of Brora, Sutherlandshire ; and on the British Equiva- 

 lents of the Neocomian System of Foreign Geologists." By Rode- 

 rick Impey Alurchison, Esq., F.G.S. 



In this communication the author confirms the interesting disco- 

 very announced by Mr. Robertson in the preceding paper, and re- 

 marks, that as the reefs of rock exposed at low water at the mouth 

 of the river Brora unquestionably lie beneath the Oxford clay, and 

 are not far above the roof of the coal, there can be no doubt that the 

 beds containing the freshwater shells, being fairly intercalated with 

 the other strata, are thus inclosed in the heart of the oolitic series. 

 They had escaped the notice of Mr. Murchison, probably from ha- 

 ving been covered by sea sand at the time of his visit. 



An examination of the freshwater specimens collected by Mr. 

 Murchison and Professor Sedgwick at Loch Stafiin, in the Isle of 

 Skye, has identified the principal forms with Mr. Robertson's spe- 

 cimens from Brora, and has led the author to ado])t a different view 

 res])ecting the position of the beds from which they were derived. 

 Instead of supposing that the oolitic series of the cliffs near Portree 

 was overlaid by a true equivalent of the Wealden*, the freshwater 

 beds of Skj^e will it is now believed be found, like those of Inver- 

 brora, to be interstratified with the middle oolite, a conclusion ren- 

 dered probable by the natural sections and form of the coast, and 

 by the circumstance that the fragments not found in situ which 

 contained freshwater shells were collected near the escarpment and 

 not on the dip of the oolitic strata. Mr. Murchison is inclined to 

 take a similar view of the freshwater deposits near Elgin, compared 

 by Mr. Malcolmson to the Purbeck beds of England. 



The author remarks, that with the terrestrial evidences in the plants 

 of Portland, Scarborough, Stonesfield and Brora, Ave might naturally 

 expect at any day to liear of the associated lacustrine or river shells. 

 But Mr. Robertson's discovery further comjiels us to believe, that the 

 same species of freshwater shells prevailed, not only during the whole 

 of the Wealden epoch, but that they were in existence at periods 

 long antecedent, when the adjacent lands poured forth rivers into the 

 sea in which the middle and lower oolites were accumulated, and 

 thus we acquire a new element to enable us to reason upon the 

 former conditions of the surface. 



The facts stated by Mr. Robertson tend to confirm the idea, that 

 the Wealden is more naturally connected witli the Jurassic than 

 with the cretaceous system, and must also have an influence in de- 

 ciding that the Neocomian formation of foreign geologists ought not 

 to be placed on the parallel of the Wealden. Mr. Murchison has for 

 some years been of opinion that the Neocomian system is little more 

 than an equivalent of the lower greensand of British geologists, a 

 view which he u])held at the meeting of the Geological Society of 

 France at Boulogne in 1839, on the ground of the identity of their 

 stratigraphical relations and typical fossils. Further researches du- 

 ring last May along the coast of the Isle of Weight, in company with 

 * Geol. Trans vol. ii. p. 30(5. 



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