Vol. 67.] WEST, MID, AND EAST SOMEESET. 



II. Subdivisions kecognizable in the Rh^tic Rocks 



OF THE DlSTEICT DESCRIBED. 



The beds that lie between the Keuper and the Lias constitute 

 the Rhaetic Series. The Series admits of dual division — into Upper 

 and Lower Rhaetic. 1 The Sully Beds and Black-Shale subdivision, 

 or the ' Zone of Avicula [Pteria] contorta ' of earlier authors, con- 

 stitute the Lower Rhsetic ; while the ' Upper Rhaetic ' of my 

 previous papers, the White Lias proper and certain immediately- 

 superincumbent marly beds (' Watchet Beds,' see p. 8), compose 

 the Upper Rhsetic as now defined. These two primary subdivi- 

 sions are very distinct one from the other, both as regards lithic 

 structure and as regards faunal characters. 



Lower Rhsetic. 



Sully Beds. — Concerning these beds almost sufficient has been 

 said. The essential points to remember are that in reality they 

 are only the topmost 14 feet or so of the Grey Marls that 

 contain Rhsetic fossils. 2 Their upper surface in places shows 

 evidence of channelling by water previous to the deposition of 

 the Black Shales, and this fact — among others — points to a non- 

 sequence, which largely explains the difference in appearance 

 between the Sully Beds and the succeeding Black Shales. 



Westbury Beds. — This term is suggested for use, as an 

 alternative term to ' Black Shales/ etc. 



The Westbury Beds vary considerably in thickness in this 

 country : at Borrow Hill, near Tewkesbury, they are but 9 feet 

 thick ; at Lilstock, 32 feet ; and at Blue Anchor, near Watchet, 

 they may be as much as 46 feet thick. In other localities they 

 are thinner, but nowhere thicker. It is interesting to note that, 

 where the Westbury Beds are thick, so are the subjacent Tea-green 

 Marls or Tea-green and Grey Marls, and vice versa. 



The Westbury Beds were deposited not far from the coast-line, 

 for this is shown by their Swabian or littoral facies. The mollusea 

 have a dwarfed appearance : gastropods are abundant in certain 

 beds in Somerset, and in the sandstone of Pylle (Glamorganshire) 

 specimens of Natica abound ; pieces of lignite are not infrequent 

 in some of the beds in Glamorgan and Somerset ; and, except for 

 Orbiculoidea townshendi (Forbes), brachiopods are wanting. 



1 [Certain other terms were suggested for these subdivisions when the paper 

 was communicated (see Discussion, p. 73) ; but, in deference to the wishes of 

 the Publication Committee, those terms are not used here.] 



2 In places in West Somerset Prof. Boyd Dawkins also found characteristic 

 Hhsetic fossils in some abundance in the Grey Marls that are now correlated 

 with the Sully Beds. (Q. J. G. S. vol. xx, 1864, pp. 397 et seqq.) 



