ROCKS OF SOMERSET AND DEVON. 393 



General Deductions. 



We find in the area last treated of a tolerably persistent series of 

 conformable strata of much greater thickness and more variable 

 composition than that of the Bridgewater district and the other 

 tracts covered by Triassic rocks south of the Mendips, whilst in the 

 area north of the Mendips they seldom exceed 200 feet. 



Hence we are justified in considering that the area east of the 

 Quantocks and of Taunton, and south of the Mendips, was dry land 

 during the deposition of the Breccias, Lower Marls, and Con- 

 glomerates in the West-Somerset and South-Devon areas, and did 

 not come within the influence of the Triassic waters till after the 

 commencement of the deposition of Upper Sandstones in the latter. 

 The question as to the correlation of the Devon and Somerset Trias 

 with that of Lancashire, Cheshire, Staffordshire, Leicestershire, and 

 Warwickshire yet remains to be solved. Whilst the Upper Marls and 

 Sandstones of the South- coast and Watchet areas, and their easterly 

 extension to the Bridge water area, and that of the former, at any 

 rate, to the southern flanks of the Mendips, leave no room for doubt 

 as to their Keuper age, and the dolomitic conglomerate and marls 

 north of the Mendips are evidently the later deposits of the Keuper 

 period, we are at a loss to account for the remaining members of 

 the South-Devon Trias. 



Taking an average of 2187 feet for the Keuper in the midland 

 counties, and 924 feet as the average thickness of the Bunter, from 

 Prof. Hull's table of thicknesses in his memoir on the Permian and 

 Trias (p. 108), we have a total of 3111, or about 460 feet under 

 the outside estimate allowed for the South-Devon Trias. Of that 

 estimate of 3580 feet, the Upper Marls and Sandstones, which are 

 evidently of Keuper age, constitute 1880 feet. 



If the remaining 1700 feet be taken as Keuper, we have an ab- 

 normal development ; but if, as appears most likely, the lower 

 divisions are of Bunter age, the absence of unconformity throughout 

 the series makes it evident that no break here took place during the 

 Muschelkalk period, but that that hitherto unfound division must 

 have its British representative in South Devon. Whether we con- 

 sider the Pebble-bed division or the Lower series of marls as repre- 

 senting the Muschelkalk, it is evident that if we consider the Lower 

 beds as Bunter, it must also be represented. 



It would be puerile were I to infer that, because no lithological 

 equivalent occurs, contemporaneous deposition did not take place, 

 and would be in the face of the evidence I have endeavoured to bring 

 forward as to the interchangeable nature of the constituents of single 

 divisions within a limited area, as, for instance, the marlstones almost 

 replacing the Upper Sandstones near Bishop's Lydeard and at 

 Cothelstone, and the different positions occupied by the sandstones 

 in the Breccia division in the north and south parts of the area. 



As every one knows, the nature of accumulating sediments depends 

 largely upon the materials whence they were derived, which 

 is well instanced in the dolomitic conglomerates of the Mendip 



