Vol. 51.] THE COUNTRY AROUND FISHGUARD. 161 



lava-stream have been more or less resorbed into a newer flow. 

 The degree of consolidation, the composition of the fragments, the 

 temperature and conditions of flow, etc., of the later lava-stream 

 which involved them, have determined the extent of their re-fusion 

 and manner of incorporation. Further details of these interesting 

 rocks will be given in the description of their microscopic 

 characters (see p. 176). 



The only published description of any of the felsitic rocks from 

 this area is found in Mr. F. Butley's Survey Memoir on 'The 

 Felsitic Lavas of England and Wales.' 1 Here he mentions a crypto- 

 crystalline rock from Llanwnda with a banded flow-structure indi- 

 cating a rhyolitic type. Another specimen, ' collected between the 

 -camp and the north coast ' of Strumble Head, lacks the banding, 

 and ' although there is no proof that it is a rhyolitic rock, its 

 microscopic character .... shows that it consists of a meshwork or 

 felting of delicate crystalline spiculae, with apparently a little iso- 

 tropic matter.' This is a bluish-grey flint-like rock, or hornstone. 



"With regard to the difficulty of distinguishing these rocks from 

 altered slates and flags, Mr. Rutley in the same memoir goes so far 

 as to say that ' it would, in fact, seem that we have passages from 

 slates and flags into felsites, so that in the absence of any structural 

 peculiarities it is scarcely possible to distinguish the one class of 

 rocks from the other.' Clifton Ward held ithe same opinion, which 

 seems, however, rather an exaggeration of difficulties. 



Mr. Teall mentions in his ' British Petrography,' p. 336, that 

 ' on the west side of Goodwick Bay, near Fishguard, there is a fine 

 exposure of banded and brecciated felsites associated with felsitic 

 agglomerates,' but he gives no further account of them. 



(2) Microscopic Characters of the Lavas. 



As we have to deal with a great diversity of types amidst the 

 lavas (felsites) of this area, it may be useful to adopt some sort of 

 classification, though it must be remembered that in nature there 

 are no sharp divisional lines between the different types. A series 

 of specimens might easily be selected in which every stage of transi- 

 tion would be represented. Teall (' Brit. Petrogr.' p. 308) roughly 

 classifies the felsites ; he says ' some felsites may be resolved into 

 microcrystalline aggregates of definitely recognizable minerals 

 (quartz and felspar or mica) ; others appear cryptocrystalline, and 

 others again are isotropic, but without that entire absence of struc- 

 ture which distinguishes a glass. In addition to the above we find 

 felsites which break up under crossed nicols into aggregates of 

 double-refracting grains or patches, often of extreme irregularity, but 

 of considerable size. The different varieties of felsite above referred 

 to are intimately related to each other, and often co-exist in the 

 same rock.' Teall has here followed the obvious plan of classifica- 

 tion by the characters of the groundmass, and to a certain extent 

 it may be conveniently adopted in this case. 



1 Mem. Ghsol. Surv. 1885, pp. 20, 21. 



