184 me. h. h. thomas on the [May 191 r* 



which it has been thought advisable to take the names skomerite 

 and marloesite. 



The main rock-types which have been mapped are the following : — 

 soda-rhyolite and felsite, albite-trachyte, keratophyre, skomerite, 

 marloesite, mngearite, olivine-basalt, and olivine-dolerite. 



The acid rocks are easily separated in the field from those of 

 more basic character ; but further subdivision on the ground is 

 difficult, on account of the fine texture of the lavas. Generally 

 speaking, however, the olivine-basalts can be detected by their 

 darker colour, finely crystalline character, and by having a purple 

 tinge imparted to them by the augite. 



It has not been possible to separate in the field the trachytes, 

 keratophyres, skomerites, and marloesites one from the other, except 

 in a few instances. As a whole, however, these rocks may be 

 separated from the rhyolites on the one hand, and the mugearites 

 and basalts on the other, by their greenish or greenish-grey colour 

 and more vesicular nature. 



(b) Their Geographical Distribution. 



The geographical distribution of the Skomer Volcanic Series as a 

 whole has, for the most part, been clearly indicated by previous 

 writers. 



The rocks near St. Ishmael's, however, were regarded as intrusive, 

 and were shown on the old edition of the Geological Survey Map 

 (O.S. Sheet 40) as crossing the strike of the sedimentary rocks 

 with which they are associated. Mr. Cantrill r has clearly demon- 

 strated their true character, and described their extent. 



In detail, the members of the Skomer Series have a simple dis- 

 tribution. The islands of the Smalls and Grassholm are too remote 

 to allow of a detailed correlation with Skomer and the mainland ; 

 but Skomer, Midland, and the mainland are directly comparable. 



A most useful datum, in addition to that of the sedimentary 

 group, is furnished by a band of red felsitic rock which occurs just 

 south of the Wick on Skomer Island, and lies 250 feet above the 

 sediments. It may be detected on the Neck and on Midland 

 Island, where, on account of its superior hardness and splintery 

 fracture, it makes conspicuous crags. 



The subdivisions as developed on Skomer may be found to a 

 limited degree on the mainland ; but the rocks are evidently thinning 

 in an easterly direction, and some of the minor divisions have 

 disappeared. The rhyolite of Musclewick (east of the map) might 

 well be on the horizon of that of the North Cliff of Skomer. It is 

 presumably succeeded by the mugearitic group of Wooltack, which 

 may represent that of the centre of Skomer: the lower basalts of 

 Skomer being unrepresented on the mainland. The keratophyres, 

 etc., below the main mass of sediments, and the basalts above the 

 sediments, are present in both areas. Minor differences between 



1 'Summary of Progress for 1909' Mem. Geol. Surv. 1910, p. 21. 



