T 



[ 465 ] 



XLYI. Proceedings of Learned Societies. 

 GEOLOGICAL SOCIETY. 

 [Continued from p. 221.] 



January 25th, 1911.— Prof. W. W. Watts, Sc.D., M.Sc, F.li.S., 



President, in the Chair. 



KE following communication was read : — 



' The Skomer Volcanic Series (Pembrokeshire).' By Herbert 

 Henry Thomas, M.A., B.Sc, JF.G.S. 



This series of volcanic rocks is developed in the west of Pem- 

 brokeshire, and takes its name from the island of Skomer, where 

 the maximum thickness is exposed. The rocks are traceable on 

 the mainland from near St. Ishmaels on Milford Haven to Wool- 

 tack Head, and on the west occupy the islands of Midland, Skomer, 

 and the Smalls. The thickness exposed is some 3000 feet, and the 

 lateral extension some 25 miles. The district described is that 

 portion of the mainland known as the Wooltack peninsula, and the 

 islands on the west. The chief evidence bearing on the age of the 

 series is obtainable from the mainland sections, and indicates that 

 the rocks are of pre-Upper Llandovery age ; but, from a con- 

 sideration of the geology of the neighbouring country, it is probable 

 that their true age is Arenig. The rocks are chieny subaerial lava- 

 flows of extreme thinness and great lateral extent; they are 

 frequently interstratih'ed with red clays, and are separated into 

 two main groups by a thick mass of sedimentary rocks barren <>1 

 fossils. There is an almost complete absence of pyroclastic rocks, 

 a fact which points to the eruptions being of the fissure-type. 



The lavas form well-defined groups in the field, and range from 

 extremely acid varieties with a silica-percentage of about 80 to 

 others with a percentage of less than 50. The rocks of the series 

 fall into eight chief types, two of which are of necessity new ; in 

 order of increasing basicity they are as follows : — soda-rhyolites, 

 soda-trachytes (including olivine- and hypersthene-bearing varie- 

 ties), keratophyres, skomer ites. marloesites, mugearites, 

 olivme-basalts, and olivine-dolerites. The last named are pro- 

 bably intrusive. All these types are described in the paper, and 

 several analyses are tabulated. The first five types may be included 

 in the alkaline class ; they are rich in soda, and most of the felspars 

 belong to albite-oligoclase varieties. The last three types are 

 normal subalkaline rocks, in which the felspars range from oligo- 

 clase to labradorite. 



The author does not consider that the Skomer rocks have been 

 albitized to a sufficient degree to mask their original characters, 

 and regards them as a mixture of alkaline rocks of Pantellerian 

 affinities, with a subordinate series of more basic subalkaline rocks, 

 the most basic of which are probably intrusive. 



