DISTRIBUTION: — BRITISH ISLES 603 



"Its surface displays a mass of spheroidal or pillow-sliaped blocks aggre- 

 gated together, each having a tendency to divide internally into prisms which 

 diverge [converge ?] from the outside towards the center." 



This occurrence is also described by Cox and Jones/^ who state that there 

 is a little associated chert, and that the rock closely resembles that of 

 Mullion Island, Cornwall. 



At the extreme west of Pembrokeshire, Jeffreys Haven, on the north 

 side of Saint Brides Bay, Thonias^^ records the occurrence of a basic lava 

 with good pillow structure in a series of quartzites and shales immedi- 

 ately beneath the Upper Llandovery shales. At Strumble Head, Pem- 

 brokeshire, occurs a variolitic rock which was formerly thought intrusive 

 and which has been described as having "spheroidal jointing." Cox and 

 Jones^^ find here a succession of much altered highly basic flows, some of 

 which have well developed pillow structure, while others show transitions 

 to nonpillowy types. There is much associated chert, especially with 

 those having a pronounced pillow structure. Typical pillows range from 

 1 foot to 18 inches in diameter. 



England. — Fox^^ first described the pillow lavas of Mullion Island, 

 south coast of Cornwall, as fine-grained greenstones 



"of a peculiar globular or ellipsoidal structure, associated in certain parts of 

 the island with bands, sheets, and lenticles of chert, shale, and limestone." 



At the same time a more detailed description of the rocks and their struc- 

 tures was presented by Fox and Teall,^* in which the greenstones are 

 characterized as 



"separated into rude rolls by curvilinear joints. The rolls show circular or 

 elliptical outlines in cross-section and measure from a few inches to 2 feet in 

 diameter. Flat surfaces of this rock, such as are exposed at many places at 

 the base of the cliff, remind one somewhat of the appearance of a lava of the 

 pahoehoe type." 



The rolls tend to a parallel elongation. Bits of limestone are found be- 

 tween them and patches of chert and shale sticking to their surfaces. 

 The stratified rocks are thin strips of cherts, shales, and limestones both 

 underlain and overlain by igneous rocks and which can not be traced 



80 A. H. Cox and O. T. Jones. Loc. cit. 



^ H. H. Thomas: The Skomer volcanic series (Pembrokeshire). Quar. Jour, Geol. Soc. 

 London, vol. 67, 1911, pp. 175-214. 



32 A. H. Cox and O. T. Jones. Loc. cit. 



33 Howard Fox : Mullion Island. Jour. Roy. Inst. Cornwall, vol. xil, 1893-1894-1895, 

 pp. 34-38. 



3* Howard Fox and J. J. H. Teall : On a radiolarian chert from Mullion Island. Quar. 

 Jour. Geol. Soc. London, vol. 49, 1893, pp. 211-220. 



