516 Notices of Memoirs — British Association — 



Abstracts of Papers read in Section C ( Geology), Meeting of British 

 Association, Birmingham, September 10-17, 1913. 



1. On tarious Occurrences of Pillow Lavas in Kokth and 

 South Wales. Bv A. Hubert Cox, M.Sc, Ph.D., F.G.S., and 

 Professor 0. T. Jones, M.A., D.Sc, F.G.S. 



Pillow lavas were described from four localities, viz. : — 



{a) Strumble Head, in Pembrokeshire. 



(b) Cader Idris, in Merionethshire. 



{c) Sarn Mellteyrn, near Pwllheli, Carnarvonshire. 



(d) Careg, 2 miles N.W.AV. of Aberdaron, Carnarvonshire, 

 (rt) Strumble Head (A. H. Cox). — References were made particularly 

 to the work of Reed and Elsden. The rocks were formerly regarded 

 as intrusive, and were described as of composite characters and 

 possibly of later date than the main folding. 



Variolitic rocks were described by Reed, who referred to the 

 'pillow structure' as 'spheroidal jointing'. The whole mass 

 appears to consist largely of highly vesicular, basic flows, some 

 with well-developed pillow structure, others showing transitions to 

 non-pillowy types. Abundant chert occurs in association with the 

 lavas, particularly those showing pronounced pillow structure. The 

 most perfect pillows vary from a foot to 18 inches in diameter, 

 and consist of typical spilites, with thin, rod-like felspars of refractive 

 index about 1-542, corresponding to oligoclase — the rocks are con- 

 siderably decomposed, especially to calcite, chlorite, and epidote. 



Among the above rocks are ophitic diabases, showing marked 

 columnar jointing, which may in part represent sills. 



(b) Cader Idris (A. H. Cox). — A thick band of pillow lavas forms 

 the highest point of the Cader Idris range, and then strikes 

 W.S.W. Its distribution is described in detail, and reference is 

 made to the work of Ramsay and Geikie. A comparison of these 

 rocks with those of Strumble Head discloses certain differences, 

 especially in their uniformly less vesicular character and smaller 

 amount of associated chert. Under the microscope the rock shows 

 the character of a typical spilite ; both rod- and lath-shaped felspars 

 occur, the former being oligoclase, the latter somewhat richer in soda 

 (refractive index below 1-541). Th^ rock is considered to resemble 

 most closely that of MuUion Island. In close association with it 

 is the ' Eurite' (soda-granophyre) of Cole and Jennings. These lavas 

 appear to occupj' a stratigraphically higher horizon than the beds which 

 yielded Didymograptus bifidm and B. murehisoni to Lake and Reynolds. 

 The detailed examination of the area is still incomplete. 



(c) Sarn Mellteyrn (0. T. Jones). — References were made to the work 

 of various aiithors, viz. Ramsay, Harker, Raisin, Elsden, Matley. 



The rocks are exposed by the roadside three-eighths of a mile south- 

 west of Mellteyrn Church, where 10 to 12 feet of typical pillow lavas 

 overlain by a similar thickness mainly of non-pillowy rocks of allied 

 characters are followed by flinty mudstones and micaceous shales. 



