186 



MR. H. H. THOMAS ON THE 



[May 1911, 



Island is also a thin mass, and in the hand-specimen shows no 

 structure other than occasional flow-brecciation. 



The Mewstone mass may be intrusive ; it succeeds a series of 

 quartzites, and its base is more glassy (fig. 3 A, below) than its 

 higher portions. It is about 100 feet thick, and shows a roughly 

 columnar structure towards the summit of the Mewstone. 



The rhyolitic rocks of Musclewick on the mainland are similar 

 to those of the North Cliff of Skomer, and are probably on the same 

 volcanic horizon. Microscopically, the rocks range from crypto- 

 crystalline devitrified obsidians to almost noncrystalline varieties 

 presenting microlitic or even trachytic structures. The devitrified 

 obsidians, such as those forming the base of the Mewstone and 

 some bands in the northernmost mass, are beautifully perlitic 

 (fig. 3 A), and occasionally, like some of the Tom's-Honse rocks, 



Fig. 3. 



A= Perlitic structure in the lowest part of the soda-rhyolite (?) of the Mewstone 



(Skomer). Slide E 71 16. 1 X 25 diameters. 

 B = Contorted lines of flow in the soda-rhyolite of Bull Hole (Skomer) ; from 



a slide kindly lent by Mr. F. T. Howard. X 25 diameters. 



both perlitic and spherulitic. Flow-structures are almost constant 

 features, and contortion of the flow-lines may be noticed both on 

 the macroscopic and on the microscopic scale (fig. 3 B). On a large 

 scale, these contortions are well exhibited above Tom's House. 



1 Where registration-numbers of slides are mentioned, these refer to the 

 slides in the collections of the Geological Survey. 



