Yol. 51.] THE COUNTRY AROUND FISHGUARD. 169 



this structure is that the rocks contain veins of secondary quartz, 

 second growths of quartz round earlier formed grains, vesicles 

 infilled with quartz, felspars replaced by quartz [330], and similar 

 evidence of the infiltration and addition of silica since the solidi- 

 fying of the rocks. Again, the associated tuffs and breccias are for 

 the most part silicified and have assumed a flinty texture, while 

 some rocks which almost certainly are tuffs show some indication 

 of micropoikilitization. Therefore the evidence on the whole might 

 seem to point to a kind of silicification, possibly brought about by 

 solfataric action, as suggested by Miss C. A. llaisin for certain rocks 

 from North Wales. 



But a most suggestive paper by Mr. W. M. Hutchings in the 

 Geological Magazine for February, 1894, p. 64, draws attention to 

 the development of a fine mosaic or ' tesselated ' structure as a 

 result of contact-metamorphism in slates. It is possible, or rather 

 probable, that we have in these felsites an analogous case : the con- 

 ditions for contact-metamorphism are most favourable, masses of 

 intrusive diabase, etc., being present in abundance. 



Mr. Hutchings puts forward the view that a kind of solution of 

 minerals must have been formed by a process resembling aqueous 

 fusion. The presence of heat, pressure, and a certain amount of 

 water is necessary, judging from laboratory experiments, but these 

 were undoubtedly present in nature ; and the highly acid ground- 

 mass of these old rhyolitic lavas, whether in a cryptocrystalline or 

 glassy condition, produced a solution in which floated the less 

 easily attacked microlites, granules, etc., of the original rock. On 

 the cooling of the mass the solution crystallized as a blurred aggre- 

 gate of quartz and felspar, forming a kind of granophyric structure 

 — the blurred mosaic. In cases in which the quartz and felspar 

 separate out from each other more completely, or in which the 

 solution is almost of pure silica (as would be likely in the highly 

 acid magma of the rhyolites forming the original undifferentiated 

 or glassy base), the mosaic is less blurred, and each patch is more 

 definitely marked out. When the solution of the groundmass was 

 only in its infancy, merely small portions of the rock were affected, 

 and if the necessary conditions for the metamorphism were abruptly 

 removed at this stage, the rock would re-solidify so as to exhibit a 

 few blurred, scattered patches of the mosaic. A similar incomplete 

 development of the mosaic may be due to the penetration of rocks, far 

 removed from the contact-area, by tongues of the highly heated acid 

 and corrosive solution in its migration from its original home. But 

 I am not inclined to think that there was much, if any, lateral trans- 

 ference of material or migration of solution from its place of forma- 

 tion ; otherwise the preservation of original structures, such as the 

 pcrlitic, and the absence of superinduced flow-structure would be 

 difficult to explain. If Mr. Hutchings's theory of solution be the true 

 explanation of the mosaic structure in the felsites, the solution must 

 have consolidated in its original home : there can have been no lateral 

 or horizontal flow ; and the rate and local conditions of cooling, and 

 the composition and structure of the rock, etc., must have been the 



