337 



into consideration the highly siliceous character of these rocks. In 

 specific gravity they range from 2 - 6 to 27 ; the average lying nearer 

 the former than the latter. The mode of occurrence of these rocks 

 is described in the Survey Memoir on North Wales. They are in 

 many cases undoubtedly lavas ; and they frequently exhibit the banded 

 vesicular, spherulitic and nodular structures so characteristic of lavas 

 of acid composition. The discovery of perlitic structure in the rocks 

 of the Glyders (Esgair-felen) and Snowdon (Clogwyn d'ur Arddu) by 

 Mr. RUTLEY has demonstrated the fact that some of them were 

 originally obsidians or pitchstones. No distinct traces of glass now 

 remain. The rocks have been completely de vitrified. Under crossed 

 nicols they give a crypto-crystalline, micro-crystalline or confusedly 

 crystalline reaction. Sometimes large irregular patches give uniform 

 extinction. The boundaries of these patches are, however, never well- 

 defined in ordinary light, and even under crossed nicols they are 

 wanting in sharpness. Granules of opacite, ferrite or viridite are 

 frequently scattered through the mass without any regard to the 

 double-refracting portions. Glassy lavas frequently show a want of 

 uniformity. Bands and intricate convolutions are often apparent owing 

 to a variation in the physical properties of the glass. These structures 

 are not seen under crossed nicols because every portion of the glass 

 appears dark. Precisely similar structures may be seen in the Snow- 

 donian felsites, as, for example, in those occurring near Pitt's Head 

 not far from the Beddgelert road. Here, however, they may be seen 

 under crossed nicols in consequence of the fact that the different 

 varieties of glass in the original rock have given rise to different 

 devitrification-structures. If we compare the felsitic lavas of the 

 Snowdonian area with the vitreous lavas of more recent geological 

 periods we note in both the occurrence of vesicular, banded, 

 spherulitic, nodular, perlitic and fluxion structures. The felsitic lavas 

 differ, however, in being stony and in possessing a higher specific 

 gravity say 2T> as against 2 '4. 



In speculating on the possible causes of devitrification it is well 

 to observe that the rocks in question have been involved in the earth- 

 movements that have so powerfully affected the district. In certain 

 localities a schistosity has been developed and sericite has been formed 

 along definite planes and in the mass of the felsite. To such an 

 extent has this proceeded that the rocks in certain places have become 

 sericitic schists/ 1 ) The term " porphyroid " is applied on the Continent 

 to felsites of all kinds which have become more or less schistose in 

 consequence of dynamic metamorphism and in which secondary mica- 

 ceous minerals, especially sericite, have been developed. The schistose 

 felsites of Wales resemble the " porphyroids " of the Ardennes &) and 



(1) See BONNEY, Q.J.G.S., Vol. XXXVIII. (1882), p. 289. 



(2) DE LA VALLEE POUSSIN ET RENABD. Menioire sur les roches ditea plutoniennes, &c. 

 A VON LASAULX. Sitz. d. Niederrheiu. Gesell. August, 1884. 



