494 MR. E. E. L. DIXON AND DR. A. VAUGHAN ON [Nov. 1911, 



Fauna. — Abundant (p. 550). 



Thickness. — At Oystermouth at least 175 feet, and possibly as 

 much as 350 feet, if, as is probable, the effects, in duplicating 

 outcrop, of the strike-fault shown on the Geological Survey map 

 between Oystermouth Castle and Colts Hill are negligible. 



Limit. — The junction with the Pendleside Series, P, is nowhere 

 clearly exposed. D 23 and P are seen together in the Bishopston 

 road-cutting, but their relations cannot be determined with 

 certainty, as both are weathered and crumbling and their steeply- 

 dipping junction strikes along, and largely beneath, the road. 

 The section suffices to show, however, that D 23 with its abundant 

 fauna is succeeded by a comparatively barren radiolarian-chert 

 group (the base of P), the change, which is great and far-reaching 

 both faunally and lithologically, being abrupt, if not marked by 

 an unconformity (see p. 529). Of unconformity, however, there 

 is no evidence, stratigraphical or palseontological, although the 

 junction contrasts strongly, in its abruptness, with the gradual 

 passage by alternation at Loughshinny. 1 



Typical exposures. — Oystermouth: — The Black Lias quarry 

 (fig. 3, p. 492) affords an excellent section of a thickness of 86 

 feet of beds in the condition of ' black lias ' from near the top 

 of D 23 downwards; the sequence is without notable lithological 

 or faunal change, and only one limestone, the thickest (3 feet), 

 is sufficiently pure to make lime. In Colts Hill northern quarry 

 the lowest beds only are exposed. ' Black lias ' also forms the 

 intervening ridge on which Oystermouth Castle stands. 



Bishopston : — In the cutting — chiefly along its south side — on 

 the Clyne Common-Bishopston Valley road, the top of D 2 _ 3 , to a 

 thickness of at least 45 feet, 2 is exposed in the condition of 

 ' rottenstones.' The sequence is obscured by weathering and 

 foundering, but has yielded a rich fauna, largely as moulds. 



P = POSIDONOMYA Zone. 



This zone is represented by the Pendleside Series, first recognized 

 in Gower by Dr. Wheelton Hind. 3 He correlated, however, with 

 this series not only the beds now under discussion, but also the 

 D 23 ' rottenstones.' The composite group, thus constituted, had 

 previously been called 'Bishopston Beds' by De la Beche 4 and 

 ' Gower Shales or Series ' by Phillips. 5 As regards the beds above 

 D 2 _ 3 , Dr. Hind's correlation has been confirmed 6 by the subsequent 



i C. A. Matley & A, Vaughan, Q. J. G. S. vol. lxiv (1908) p. 413. 



2 Details are recorded by Mr. Tiddeman, Swansea Memoir, pp. 22, 24. 



3 Geol. Mag. 1902, p. 485 ; ibid. 1904, pp. 402, 585-87. 



4 Mem. Geol. Surv. vol. i (1846) pt>. 133-34. 

 s ' Manual of Geology ' 1855, pp. 169-70. 



6 Of the fauna quoted by him, Glyphioceras diadema (Beyrich) alone may be 

 regarded as of zonal value ; and confirmation is, therefore, important. m 



