I 20 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



The volcanic products discharged from these vents consist of a 

 succession of lava-streams separated by bands of slate, tuff, con- 

 glomerate, and breccia. These fragmental intercalations, which 

 vary from a few yards to many hundred feet in thickness, are im- 

 portant not only as marking pauses in the emission of lava or in the 

 activity of the volcanoes, but as affording a means of tracing the 

 several lavas to their respective vents. Essentially, however, the 

 volcanic materials consist of lava-flows, the intercalations of fragmen- 

 tary materials, though numerous, being comparatively thin. The 

 thickest accumulation of tuffs is that forming much of the upper part 

 of Snowdon. It is set down by my predecessor at 1200 feet in thick- 

 ness, but I should be inclined to reduce this estimate considerably. I 

 shall have occasion to show that the summit and upper shoulders 

 of Snowdon are capped with andesites interstratified among the 

 tuffs. Sir Andrew Ramsay has referred with justice to the difiiculty 

 of always discriminating in the field between the fine tuffs and 

 some of the lavas*. Yet I am compelled to admit that, if the 

 ground were to be re-mapped now, the area represented as covered 

 by fragmental rocks would be considerably reduced. Mr. Harker 

 is undoubtedly correct when he remarks that, taken " as a whole, 

 the Bala Volcanic Series of Caernarvonshire is rather remarkable for 

 the paucity of genuine ashes and agglomerates " f. 



The lavas of the Bala volcanic group, like those of the Arenig series, 

 were mapped by the Survey as " porphyries," " felstones," or " fel- 

 spathic traps." They were shown to be acid-lavas, having often a 

 well-developed flow-structure comparable with that of obsidian and 

 pitchstone, and to consist of successive sheets that were poured out 

 over the sea-floor. Their petrography has subsequently been studied 

 more in detail by many observers, among whom I need only cite 

 Professor Bonney, Professor Cole, Mr. Rutley, and Mr. Teall ; the 

 most recent, as well as the most important, additions to our 

 knowledge of this subject having been made by Mr. Harker in 

 the Essay to which I have already referred. 



The great majority of these lavas are thoroughly acid rocks, and 

 present close analogies of composition and structure to modern 

 rhy elites, though I prefer to retain for them the old name of "felsites." 

 Their silica percentage ranges from 75 to more than 80. To the 

 naked eye they are externally pale greyish, or even white, but when 

 broken into below the thick decomposed and decoloured margin 



* Op. cit. p. 148. 



t * Bala Volcanic Rocks,' p. 25. 



