Proceedings of the British Association. 349 



vents the combination is not sufficiently powerful to form any other. 

 This explanation is corroborated by the fact, that it requires a very 

 great proportion of the olefiant gas to produce the effect. 

 Mr. Herapath then read a paper on Arsenical Poisons. 



Section C. — Geology and Geography. 



The first paper was "A Classification of the Old Slate Rocks of 

 Devonshire, and on the true position of the Culm Deposits of the 

 central portion of that country," by Professor Sedgwick and Mr. 

 Murchison. — The authors began by observing, that this was a mere 

 outline of a more detailed memoir on the physical structure of De- 

 vonshire, which they were about to lay before the Geological Soci- 

 ety of London. In the published geological maps of that county, 

 the whole system of the older slate rocks was represented under 

 one color, without any attempt at subdivision ; and one color also 

 represented different limestones, 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 compare them with corresponding de- 

 posits in other places. They also wished to determine the true 

 place of the remarkable carbonaceous deposits of central Devon, 

 which had been previously regarded as belonging to the lowest por- 

 tion 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 southwest of Oakhampton. A diagram of 

 the section will be found on the succeeding page. 



In the ascending order this section exhibits — 1. A system of 

 slaty rocks, containing a vast abundance of organic remains, gene- 

 rally in the form of casts. These rocks sometimes pass into a fine 

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

 hard quartzose flagstone, not unusually of a reddish tinge; sometimes 

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

 rent 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 charac- 

 terized by hard thick-bedded red sandstone, and red micaceous flag- 

 stone, subordinate to which are bands of red, purple, and variega- 

 ted shales. The red color occasionally disappears, and the formation 

 puts on the ordinary appearance of a coarse, siliceous grauwacke, 

 subordinate to which are some bands of imperfect roofing slate. In 



