TRANSACTIONS OP THE SECTIONS. 9S 



A letter was read from Dr. Thaill referring to some specimens of 

 fossil fishes from the Caithness schist of the Island of Pomona (Ork- 

 neys), and from Clashbennie, which were exhibited to the Meeting. 



A Classification of the old Slate Rocks of the North of Devonshire, and on 

 the true position of the Culm Deposits in the central portion of that 

 County. By Professor Sedgwick and Mr. MuaciiisoN. 



The authors began by observing, that this was a mere outline of a more 

 detailed memoir on the physical structure of Devonshire, which they 

 were about to lay before the Geological Society of London. In the 

 published geological maps of that county the whole system of the 

 older slate rocks was represented under one colour, without any at- 

 tempt at subdivision ; and one colour also represented different lime- 

 stones, without any discrimination. The object of the authors was to 

 remedy these defects, to ascertain and represent the true position of 

 the successive deposits and their natural subdivisions, so as to com- 

 pare them with corresponding deposits in other places. They also 

 wished to determine the exact place of the remarkable carbonaceous de- 

 posits of central Devon, which had been previously regarded as belonging 

 to one of the lowest portions of the grauwacke formation. 



A section was exhibited of part of that county, from the north coast 

 to one of the granite peaks of Dartmoor immediately south-west of 

 Oakhampton. 



In the ascending order this section exhibits— 



1 . A system of slaty rocks, containing a vast abundance of organic 

 remains, generally in the form of casts. These rocks sometimes pass 

 into a fine glossy clay slate, with a transverse cleavage ; sometimes 

 into a hard quartzose flagstone, not unusually of a reddish tinge ; some- 

 times into a reddish sandstone, subordinate to which are beds of inco- 

 herent shale. In North Devon they are very rarely so calcareous as 

 to be burnt for lime, but in South Devon rocks of the same age appear 

 to be much more calcareous. 



2. A series of rocks characterized by masses of hard, thick-bedded 

 red sandstone, and red, micaceous flagstone, subordinate to which are 

 hands of red, purple, and variegated shales. The red colour occasion- 

 ally disappears, and the formation puts on the ordinary appearance of 

 a coarse, silicious grauwacke, subordinate to which are some bands of 

 imperfect roofing slate. In this series are very few organic remains. 

 It is several feet in thickness, occupying the whole coast from tlie west 

 end of the Valley of Rocks to Combe Martin. 



3. The calcareous slates of Combe Martin and Ilfracombe, of very 

 great aggregate thickness, abounding in organic remains, and contain- 

 ing in a part of their range at least nine distinct ribs of limestone burnt 

 for use. This limestone is prolonged into Somersetshire, and appears 

 to be the equivalent of that on thefl<mks of the Quantock Hills. 



4. A formation of greenish and lead-coloured roofing slate of great 

 thickness, and occupying a well-defined zone in North Devon, its upper 

 beds alternating with and gradually passing into a great deposit of sand- 



