TRANSACTIONS OF SECTION CO. 495 
8. On various Occurrences of Pillow Lavas in North and South Wales. 
By A. Huperr Cox, M.Sc., Ph.D., F.G.S., and Professor O, T. 
Jones, M.A., D.Sc., F.G.S. 
Pillow lavas were described from four localities, viz. :— 
(a) Strumble Head, in Pembrokeshire. 
(6) Cader Idris, in Merionethshire. 
(c) Sarn Mellteyrn, near Pwllheli, Carnarvonshire. 
(d) Careg, two miles N.N.W. of Aberdaron, Carnarvonshire. 
(a) Strumble Head (A. H. C.).—References were made particularly to the 
work of Reed and Klsden. 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 struc- 
ture’ 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 eighteen 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 considerably decomposed, especially to 
calcite, chlorite, and epidote. 
Among. the above rocks are ophitic diabases, showing marked columnar 
jointing, which may in part represent sills. 
(6) Cader Idris (A. H. C.).—A thick band of pillow lavas forms the highest 
point of the Cader Idris range, and thence strikes west-south-west. Its dis- 
tribution was described in detail, and reference was 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). The rock is considered to resemble most closely that of Mullion 
Island. In close association with it is the ‘Hurite’ (soda-granophyre) of Cole 
and Jennings. These lavas appear to occupy a stratigraphically higher horizon 
than the beds which yielded Didymograptus bifidus and D. murchisoni to Lake 
and Reynolds. The detailed examination of the area is still incomplete. 
(c) Sarn Mellteyrn (O. T. J.).—References were made to the work of various 
authors, viz.: Ramsay, Harker, Raisin, Elsden, Matley. 
The rocks are exposed by the roadside three-eighths of a mile south-west of 
Mellteyrn Church, where ten to twelve 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. The spaces between the pillows are 
occupied by closely jointed dark-grey chert. The sediments dip to the east at 
a moderate angle, and probably pertain to the lower part of the Arenig. The 
pillow lava is finely vesicular and considerably decomposed; the felspar is 
oligoclase and forms lath-shaped microlites. From its structure and mineralo- 
gical character the rock is referred to the spilitic suite. 
(d) Careg, near Aberdaron (O. T. J.).—These rocks have been described in 
detail by Raisin, and the pillow structure noted as ‘spheroidal structure.’ The 
present notes are intended to supplement that description in certain respects. 
The pillow structure is seen near Careg quarries and near the coast; individual 
pillows have a length of about two feet, and are composed of a fine-grained rock 
with small vesicles. The felspars have the extinction-angle and refractive 
index of oligoclase-albite, and are highly charged with decomposition products. 
These rocks are undoubted spilites, and were claimed as such by Dewey and 
Flett; they are associated with ‘limestones’ of a peculiar character, together 
with beds and strings of jasper, which in places wraps round the pillows in the 
same manner as the chert near Sarn. The association of that rock with a pillow 
lava may perhaps be regarded as confirming Greenly’s suggestion that the jaspers 
of Anglesey were originally cherts. The associated rocks at Careg have an 
extraordinarily complicated structure, and probably belong to the pre-Cambrian. 
