Relations of the Rhcetic Beds in Somersetshire. 201 



of h'y." These appearances have been referred to by some as eroded 

 surfaces. The structure of the stone, however, forbids such a notion, 

 as thin layers may often be split off, which correspond to the irre- 

 gular surface. Indeed, as Edward Owen,' the man who first de- 

 scribed the Landscape Marble, pointed out more than a century ago, 

 " the rough coat itself is composed of a great many thin coats laid 

 over one another." It is the same irregularity, as it seems to us, 

 which on a larger scale is seen so often in the limestones of the 

 Lower Lias — a structure probably induced during the drying and 

 consolidation of the deposits. The Gotham Marble, as is well 

 known, occurs in isolated masses. 



Professor Eamsay has pointed out the formation of the New Ked 

 Marl in an inland salt lake, and how the same general geographical 

 features ushered in the EhEetic series ; subsequently a gradual sinking 

 of the area caused a partial influx of the sea over shallow bottoms. 

 Marine forms thus migrated from a true Ehsetic ocean, but during a 

 long period shallow water under brackish semi-estuarine conditions 

 prevailed, until the conclusion of Ehsetic times, when the purely 

 marine conditions of the Lower Lias period predominated.^ 



Thus we have a clear explanation of the fauna of these times, and 

 can understand the migration into the area of Ammonites and other 

 Cephalopoda, which in this country are the chief pal^ontological 

 features which distinguish the Lias from the Ehsetic beds. 



We have not yet sufficient material for tracing out the probable 

 boundaries of the Eheetic formation in the British area. In several 

 places we find evidences of old coast margins,^ and it remains to be seen 

 whether possibly some of the Dolomitic Conglomerates and breccias 

 of the Mendips, instead of all being regarded as beach deposits of 

 the Keuper period, may not be regarded as of newer date.'^ De la 

 Beche^ remarks that "in some localities we scarcely know where to 

 draw the line between the Dolomitic and Lias Conglomerates," which 

 is accounted for by a continuation of " the same general physical 

 causes for the production, accumulation, and consolidation of the 

 gravel and fragments." 



That so distinctive a name as Ehaetic be given to these beds, and 

 that they should be mapped separately, are points which are not 

 seriously affected by our opinions as to their relations. They may 

 be conveniently regarded as a stage connecting the Keuper forma- 

 tion with the Lias, and belonging as much to the one as to the other, 

 — a stage in the history of the area under consideration which, com- 

 mencing with the conglomerates, sandstones, and marls of the Keuper 



1 Observations on the Earths, Eocks, Stones, and Minerals about Bristol, etc., 

 1754, p. 163. 



2 Quart. Journ. Geol. Soc, vol. xxvii., p. 189. See also Memoir of Edward 

 Forbes, by "Wilson and Geikie, p. 418. 



3 At Nempnet, near Chew Stoke, I mapped, in 1867, some beds of Conglomerate 

 as of Rhsetic age. — H. B.W. 



* The idea was first suggested to us in 1868 by Mr. Gibbs, late of the Geological 

 Survey. It is also held by Mr. Moore, in his paper on the Geology of the 

 Mendips, Proc. Somerset Arch, and Nat. Hist. Soc, vol. xv. 



* Mem. Geol. Survey, vol. i., p. 272. 



