Vol. 67.] SKOMER VOLCANIC SERIES. 1S"> 



the various districts would be expected, but in general the agree- 

 ment is fairly close. 



With regard to the mode of occurrence and distribution of the 

 rock-types, I have little to add to what has been admirably put 

 forward by Messrs. Howard & Small and to what may be gathered 

 from a study of the appended map (PI. XI). 



(c) The Rarity of Tuffs and Agglomerates. 



As is far from unusual in a volcanic series of this nature, pyro- 

 clastic rocks are of exceptional occurrence, 1 and, when they do 

 occur, they are chiefly associated with the more siliceous rocks. On 

 Skomer Island a thin series of rhyolitic tufts, well bedded and 

 reaching 8 feet in thickness, underlies the trachyte of Pigstone 

 Bay, but is not present on the eastern side of the island. 



A zone of rhyolitic breccia in the north-east of Skomer, called 

 by Mr. Small (1899-1909) the Way bench Breccias, is presumably 

 on the horizon of the rhyolite of the Table (op. cit. p. 10) and 

 represents the breaking up of the flows in an easterly direction. 

 Similar brecciation is noticed in the pink felsitic rock on the Neck 

 and Midland Island (below). There is almost complete absence of 

 any tuffs of intermediate character ; but traces may be observed, 

 on the surfaces of many flows, in the form of highly siliceous flinty 

 patches of clastic material occupying small hollows in the lava- 

 surfaces. Such patches may be studied in Welsh Way and on the 

 Neck. Sections indicate that the ash has undergone much silicifi- 

 cation. On the mainland a thin series of tuffs may be seen in the 

 cliffs on the north side of Cable Bay, below the quartzites. 



Y. The Petrography of the Volcanic Pocks. 

 (a) The Soda-Rhyolites and Felsites. 



The rhyolitic rocks of Skomer and the mainland are, for the most 

 part, pale as regards colour, varying from white to cream ; but occa- 

 sionally they have a distinct violet tinge, such as is observable in the 

 flows which form the half-tide rocks between the Garland Stone and 

 the North Cliff of Skomer. The rocks of the North Cliff are often 

 beautifully banded, but seldom spherulitic ; those of Manies Island 

 are both spherulitic and vesicular. 



The flows of Tom's House and the Basin are spherulitic and 

 banded rhyolites, of which the altered spherulites attain considerable 

 size. The rock at the Table is thin and composed of three bands, 

 a white band between two of darker hue. To the south towards 

 the Spit the light band appears to split into three, its central portion 

 being darker. Part of this series was considered by Mr. Small to 

 exhibit pyroclastic characters (1899-1909, p. 5). 



The red rock of the Wick. Kittiwake Cave, the Neck, and Midland 



1 A. Harker, ' Tertiary Igneous Rocks of Skye' Mem. Geol. Sury. 1004, 

 n. 15, 



