258 Prof. Sedgwick mid Mr. Murchison on the Classi€catio n 



nary lithological aspect almost entirely obliterated; the rocks 

 which occupy the place of the soft mudstones and argillaceous 

 limestones of Salop and Hereford, &c. have become hard, 

 siliceous sandstones, with a slaty cleavage; and affording no 

 evidences of clear subdivisions, they can be divided into 

 broad groups by the help alone of a veryfe-iSo of the same fos- 

 sils which teem in the great area of the Silurian region. As 

 Pembrokeshire, lying as it does between the Silurian region 

 and Devonshire, prepares us, by the peculiar structure of its 

 coal or culm-field, to recognize an almost perfect analogy of 

 structure in the culm-field of Devon; so do the great changes 

 which the underlying old red and Silurian systems have there 

 undergone, predispose us to believe that the strata which sup- 

 port the culm-field in Devon may present themselves also 

 under a peculiar mineral aspect. Notwithstanding, however, 

 this caution as to the possibility of some lower members of 

 the Devonian and Cornish strata representing Silurian rocks, 

 we adhere to the conviction, that the great mass of the strata 

 which support and appear to pass upwards into the culm-field 

 are the equivalents of the old red system properly so called. 



Instead of thinking ourselves rash and hasty in making the 

 generalization above given, we would rather accuse ourselves 

 of being tardy and over-cautious; and we are now surprised 

 (notwithstanding the imperfection of our evidence) that we so 

 long retained the older rocks of Devon and Cornwall in the 

 place where we classed them on our return from these coun- 

 ties in the autumn of 1836. For our conclusions, excepting 

 their generality, are not entirely new. Some of the red sand- 

 stone groups, at least, of South Devon, have often been called 

 old red sandstone; and they are so regarded by Mr. Austen ; 

 who also considers the Torbay limestone as the equivalent of 

 the mountain limestone. Mr. Greenough, many years since, 

 pointed out the extreme difficulty of separating the Plymouth 

 limestone from the mountain limestone ; and Mr. Lonsdale, 

 a considerable time since, believed that the system of South 

 Devon would at length be proved only a peculiar development 

 of the old red sandstone; and he freely stated this opinion 

 to Mr. De la Beche as well as to ourselves. The present 

 opinions of Mr. De la Beche are before the public, and we 

 have no right, perhaps, to be his interpreters. He puts forth 

 several hypotheses, without positively adopting one of them. 

 He must, however, (after the recent publication of such large 

 groups of Silurian fossils) before long perceive that the for- 

 mations of South Devon not merely contain fossils approach- 

 ing those of the mountain limestone (a fact long known), but 

 that their whole suite of fossils is intermediate between those 



