FOR THE ADVANCEMENT OP SCIENCE. 129 



of the Culm Deposits of the central portion of that County, by Pro- 

 fessor Sedgwick, and R. J. Murchison, Esq. The latter observed he 

 was about to submit a mere outline of a more detailed memoir 

 on the physical structure of Devonshire, which, in conjunction with 

 Mr. Sedgwick, he purposed to lay before the Geological Society of 

 London. One object they had in view was, to remedy the defects 

 in existing geological maps, as to colouring sub-divisions of form a- 

 ations : and another to ascertain, by actual sections, the true posi- 

 tion of successive deposits, and their natural subdivisions, so as to 

 bring them into comparison with other corresponding deposits, and 

 to determine their true place in the succession of British forma- 

 tions. By help of a section, the following succession of deposits in 

 the ascending order was determined : — 



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

 mains, generally in the form of casts ; these rocks sometimes pass into a fine 

 glossy clay slate, with a true transverse cleavage — sometimes into a hard 

 quartzoice flagstone, not unusually of a reddish tinge, sometimes into reddish 

 sandstone, subordinate to which are bands of incoherent shale. In North 

 Devon they are very seldom so calcareous as to be burnt for lime, but in 

 South Devon rocks of the same age appear to be much more calcareous. 

 This series is finely exposed in the Valley of Rocks and the Valley of the 

 Lyn, but its base is no where visible in this line of section. 



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

 red sandstone and red flagstone, subordinate to which are bands of red, pur- 

 ple, and variegated shales ; the red colour occasionally disappears, and the 

 formation puts on the ordinary appearance of a coarse silicious grey wacke, 

 subordinate to which are some bands of slate, but too imperfect to be used 

 for roofing. This specimen contains very fine organic remains ; it is several 

 thousand feet in thickness, occupying the whole coast from the west end of 

 the Valley of Rocks to Combmartin, being thrown back by a dip into the 

 cliffs between Portlock Bay and Linton ; it re-appears in North Hill and the 

 Quantock Hills. 



3. The calcareous slates of Combmartin and Ilfracombe, of very great ag- 

 gregate thickness, abounding in organic remains, and containing, in a part of 

 its range at least, nine distinct ribs of limestone, burnt for use. This lime- 

 stone is prolonged into Somersetshire, and is apparently the equivalent of 

 the limestone on the flank of the Quantock Hills. 



4. A formation of lead coloured roofing-slate, of great thickness, and occu- 

 pying a well-defined zone in North Devon, its upper bed alternating with, 

 and gradually passing into, a great deposit of green, grey and purple, or red 

 sandstone, and micaceous flagstone. These silicious masses alternate with 

 incoherent slates, and are in some places surmounted by great masses of red 

 unctuous shale, which, when in a more solid form, generally exhibit a cleav- 

 age oblique to the stratification. 



5. The silurian system, resting conformably on the preceding, of great 

 thickness, on the western coast of North Devon, occupying a zone several 

 miles wide, and containing many subordinate beds and masses of limestone. 

 In its range towards the eastern part of the county it gradually thins off, 

 but its characters are well preserved, and throughout it contains an incredi- 

 ble number of characteristic organic remains. 



6. The carbonaceous system of Devonshire. This system is very greatly 

 expanded, stretching in a direction E. and W. across the county, occupying 

 the whole coast from the neighbourhood of Barnstaple to St. Gennis, in 

 Cornwall, and on its southern boundary ranging so close to Dartmoor that 

 its lower beds have been tilted up and mineralized by the action of the gra- 

 nite. This great formation is, therefore, deposited in a trough, the northern 



VOX/. V. NO. XVII. It 



