252 Prof. Sedgwick and Mr. Miirchison on the Ciassi/icafwn 



by its lithological structure, tliat he classed a large portion of 

 it as primary sa7idsto7ic, — an opinion which was afterwards cor- 

 rected by an examination of that region by ourselves*. Again, 

 it has fallen to us to show, that the black bituminous schists 

 of Caithness and the Orkneys form an integral part of the old 

 red sandstone: and what trace of lithological resemblance, 

 we would ask, is there between such rocks as these, which 

 occupy the north-eastern extremity of our island, and the 

 great masses of the same deposit which prevail in the form of 

 the variegated marls and concretionary limestones of Here- 

 fordshire and Brecknockshire? Passing from the latter di- 

 strict into Pembrokeshire, we perceive the old red sandstone 

 rapidly changing its aspect and composition. The concretion- 

 ary limestones disappear ; and a hard, brownish-red, schistose 

 rock replaces the soft marls, and large tracts are occupied by 

 yellow and gi'ey, hard, siliceous sandstones. 



As, therefore, such great lithological changes actually oc- 

 cur in continuous strata within such a limited area, why may 

 we not believe (particularly when we have strong collateral 

 reasons for doing so depending both on fossils and sections,) 

 that the older fossiliferous rocks of Devonshire and Cornwall 

 are the equivalents of the old red sandstone ? They have 

 indeed, to a great extent, assumed a new mineral type — we 

 say to a great extent; for we ourselves, in both our pre- 

 vious memoirs, have described many of the rocks in question 

 as much resembli7ig the old red sandstone. 



Various causes may have co-operated in producing the pecu- 

 liar mineral character of the Devonian and Cornish strata. 

 Among these, igneous action is the most apparent, and the 

 country it is well known bristles with rocks of igneous origin, 

 many of which are well described by Mr. De la Beche: and if 

 (as we believe) he is correct in supposing that many of these 

 rocks were formed contemporaneously with the strata among 

 which they appear, we are furnished with one of the conditions 

 under which the mineral character of a part of this region of 

 the old red system has been greatly modified. But, independ- 

 ently of any supposed igneous action, what is there among the 

 analogies of sedimentary deposits which should not lead us to 

 embrace the view, that formations of the same epoch may 

 have completely distinct mineral types? Has it not been over 

 and over again demonstrated that the limestones of any par- 

 ticular series are often represented by siliceous sandstones, even 

 at the opposite extremities of our island, though the order in 

 which the organic remains occur is precisely the same in these 

 different rocks ? The sandy and shaly strata of the coast of 



• See Geol. Trans., vol. ii. p. 125. 



