Vol. 67.] SK03IER VOLCANIC SERIES. 1 83 



Below the Extensus Zone, from the Nesuretus Beds of Hicks to the 

 base of the Cambrian, there seems no place for a volcanic scries of 

 such magnitude ; nor in the pre-Cambrian rocks of St. David's or of 

 the Hayscastle 1 district are there any rocks that have much in 

 common with those of the Skomer Series. Such as the evidence is, 

 therefore, it seems to point to the series being of Arenig age and 

 on the same horizon as the rocks of Trefgarn, and of Llangynog 

 in Carmarthenshire, 2 that is to say, probably rather low down in 

 the Didymograptus-extensus Zone. 



IV. The Volcanic Hocks. 



The igneous rocks of the Skomer Series as developed on Skomer 

 Island, where the greatest thickness can be studied, consist almost 

 entirely of groups of lava-flows of varying composition, unaccom- 

 panied by dyke-rocks of any description, and penetrated only by a 

 few basic sills. 



The subaerial character of the flows seems to be proved by 

 the frequent occurrence of beds of red clay ; by the red staining 

 of many of the scoriaceous and highly vesicular surfaces ; by the 

 conglomeratic layers at the top of certain flows ; and also by the 

 character of the associated coarse sediments. 



The thickness of rock exposed on Skomer exceeds 2900 feet, and 

 even then neither the base nor the summit of the Series is reached : 

 all this is built up of flows which seldom attain 10 feet in thick- 

 ness, and are often thinner, except in the case of the more acid 

 rocks. 3 



That the eruptions were of the fissure-type is suggested by the 

 general absence of pyroclastic rocks, and by the great lateral extent 

 of individual flows compared with their thickness. The vents lay 

 probably to the west of Skomer, as is indicated by the thinning 

 of the series and the more rapid alternation of types in an easterly 

 direction. Most of the rhyolites and the dolerites seem to thin out 

 eastwards, and whole groups represented on Skomer are missing on 

 the mainland. 



(a) The various Rock-Types. 



The rock-types previously said to be represented in the Skomer 

 Series are the basalts, andesites, porphyrites, felsites, and devitrified 

 obsidians of Rutley, Teall, and Howard & Small. 



Through the kindness of Messrs. Howard & Small I have been 

 enabled to work at the large number of rock- slices which they had 

 had made, and in addition have studied seme 250 slices of rocks 

 collected during the recent survey. It has thus proved possible to 

 separate the rocks according to eight distinct types, for two of 



1 These rocks have lately been mapped by Prof. O. T. Jones and myself. 



2 T. C. Cantrill & H. H. Thomas, Q. J. G. S. vol. Ixii (1906) p. 223. 

 :! J. J. H. Teall, ' British Petrography ' 1888, p. 224. 



