514 Geological Society. 



during the production of the lower part of the (new) red sandstone 

 series of Devonshire*. 



The author endeavours, in the first place, to point out the relative 

 geological age of the red sandstones and conglomerates with which 

 these trappean rocks are associated in Southern Devon, by showing, 

 that in their continuation to the northward, along the skirts of the 

 grauwacke to the shores of the Bristol Channel, they pass into a series 

 of beds which is crowned by magnesian limestone and conglomerate, 

 equivalent to the magnesian limestone and conglomerate of the Men- 

 dip Hills and the vicinity of Bristol. The beds beneath the magnesian 

 conglomerate, which very rarely passes into a magnesian limestone, 

 from the absence of pebbles derived from older rocks, consist, for the 

 most part, of red or claret-coloured sandstones, with an occasional 

 seam or bed of conglomerate, the cementing matter of which is not 

 calcareo- magnesian. Their thickness is necessarily variable, from the 

 uneven surface of grauwacke, upon which the sandstones rest uncon- 

 formably ; but it amounts to about one hundred and fifty feet in the 

 vicinity of Wiveliscombe. The author points out, by the aid of sec- 

 tions, that the magnesian conglomerate may readily rest upon the 

 grauwacke, and conceal the lower red sandstone series by over- 

 lapping it, and that therefore it becomes exceedingly difficult to ob- 

 tain an average thickness of these lower red sandstones, which, if 

 we consider the magnesian conglomerates of the Mendip Hills as an 

 equivalent of the zechstein of Germany, would be equivalent to the 

 rothe todte liegende of the same part of Europe, and therefore be of 

 the same geological age as the lower red sandstones of the North 

 of England described by Prof. Sedgwick, and the beds noticed by 

 Mr. Murchison. 



Having thus obtained the relative geological age of the beds 

 with which the trappean rocks are associated, the author proceeds 

 to point out the occurrence of beds of sand among the more common 

 red sandstone, which presents every appearance of having been vol- 

 canic sands ejected from a crater, and which became subsequently- 

 mixed with common detrital matter then depositing. It is stated that 

 though the trappean rocks may sometimes be seen, as in the vicinity 

 of Exeter, to rest as if they had overflowed the grauwacke which the 

 (new) red sandstone series skirts, they are generally separated from 

 the grauwacke by conglomerates, or sandstones, which do not contain 

 the detrital remains of trappean rocks. Hence the author considers 

 that the (new) red sandstone series of the district generally was, to a 

 certain extent, in the course of formation when the volcanoes came 

 into activity*. 



A description is given, accompanied by a section, of the manner in 

 which the trappean rocks of Washfield, near Tiverton, are associated 

 with the (new) red sandstone of the same locality, and it is inferred 

 from the facts detailed, that events there succeeded each other in the 

 following order : 1. An original subaqueous valley or depression in 

 the grauwacke. 2. A deposit of detrital matter derived from the 



* [See note on next page. — Edit.] 



