4 ME. L. RICHARDSON ON THE RH^ETIC OF [Feb. 1911^ 



of slow formation, for Prof. Boyd Dawkins speaks of marks resem- 

 bling sun-cracks and annelid-tracks in what I identify with the 

 Sully Beds on the Watchet coast. 1 



So far as can be deduced from a study of the lithic and faunal 

 characters of the lower beds of the Black-Shale division and their 

 relationship to the Keuper Marls, the area of sedimentation in 

 closing Triassic times seems to have become continuously smaller 

 and to have been situated in the South- West of England. The 

 commencement of the changes in surface-level, which initiated 

 Rh&3tic conditions, are probably indicated by the occurrence of 

 black marls in the Grey Marls; while, just before crust- movements 

 effected the restriction of the area of sedimentation to the neigh- 

 bourhood of the present Bristol-Channel littoral, the pioneers of the 

 Rhsetic fauna must have obtained entrance. Their scanty remains 

 are preserved in the Sully Beds, and the period of minimum sub- 

 mergence is probably marked by the runnelled surface of the Sully 

 Beds and the closely-packed layer of specimens of Pteria contorta. 

 Gradual submergence then set in, and the neighbourhood of the 

 present Bristol Channel received the earliest deposits of the Black- 

 Shale division. 



Thus, while over the greater part of England the lower limit of 

 the Ehsetic Series is sharply denned, because the Sully Beds 

 and a greater or less thickness of the Black-Shale 

 division are absent, and there is therefore a non- 

 sequence, on the Bristol-Channel littoral it is most unfortunately 

 indefinite — being determined by the downward range into the 

 Grey Marls of Khaetic fossils. Eor scientific purposes, the Sully 

 Beds must be grouped with the Rhsetic ; but for cartographical 

 purposes — as I have before endeavoured to make clear 2 — they 

 must be grouped with the Keuper Marls. 



As regards the upper limit of the Rhaetic Series, in most parts of 

 the South-West of England it is fairly definite. The Lower Liassic 

 beds which are generally grouped together as the Ostrea Beds are 

 readily recognized, and underlying them is often present a band of 

 hard ' paper-shales." The Paper-Shales are not always present ; 

 but the Ostrea Beds have a wide geographical extent, and rest in 

 different places upon different members of the underlying Rhsetic 

 Series. Thus, near Wickwar, they rest upon the Cotham Marble ; 

 but in the Charlton-Mackrell district there is very clearly interposed 

 between them and the Cotham Marble the White Lias proper; at 

 Blue Anchor Point in West Somerset, both the reduced equivalent 

 of the White Lias proper and certain marly beds (' Watchet 

 Beds,' see p. 15) above ; while at Camel Hill, Moore's Insect- and 

 Crustacean Beds come in at a higher horizon. 



There is thus generally a non-sequence between the Rhsetic and 

 the Lias, which implies that, as a rule, the two series are well 

 marked off one from the other. 



1 Q. J. G. S. vol. xx (1864) p. 398. 



2 Ibid. vol. lxi (1905) p. 414 ; and Proc. Geol. Assoc, vol. xix (1906) p. 403. 



