_ Devonshire, and on the true position of the Culm Deposits of the 
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.—GeoLocy anp Grocrapuy. 
_ The first paper was “A Classification of the Old Slate Rocks of 
“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 caleareous.—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 
