Cole — The Variolite of Ceryg Oivladys, Anglesey. 115 



The fact that the variolitic crusts have escaped this replacement 

 is only another evidence that natural glass, while it may devitrify 

 by a rearrangement of its molecules into crystalline groups, is 

 none the less remarkably stable against the attacks of permeating 

 waters. The included spherulites and crystals in pitchstones and 

 obsidians are again and again found to be hollowed out, or replaced 

 by pseudomorphs in quartz or caloite, and this even while the 

 glass remains absolutely fresh. At Ceryg Gwladys considerable 

 mineral changes occurred in the tachylytic crusts when they were 

 subjected to earth-pressures ; but we may well believe that the 

 silicification of the masses on which they had developed took place 

 at an early period, while the crusts remained still vitreous and 

 glancing. 



The variolite-crust in the present hydrated condition of its 

 constituents has a specific gravity of 2'71 ; a typical specimen, 

 with similarly small spherules, from the ridge of Le Chenaillet, 

 Hautes Alpes, gives as much as 2-91. These, and the other 

 determinations quoted, have been made with a "Walker's Balance 

 in the Geological Laboratory of the Royal College of Science, 

 Dublin. 



The partial replacement of a basic lava by infiltrations of 

 silica is a far rarer occurrence than its replacement by carbonate 

 of lime ; in the latter case, moreover, the rock becomes broken 

 down, and we have a plexus of calcite veins rather than a massive 

 pseudomorph. But the late Mr. Charles Darwin has described a 

 case, remarkably similar to that of Ceryg Grwladys, as occurring 

 in the Island of Ascension.^ The jasper in this instance is found 

 " blending into the semi-decomposed basalt," and occurring " in 

 angular patches, which clearly do not occupy pre-existing hollows 

 in the rock." This observation was explained by Mr. Darwin 

 " on the supposition that a fluid removed in those parts where 

 there were no cavities, the ingredients of the basaltic rock, and 

 left in their place, silica and iron, and thus produced the jasper." 

 " I cannot doubt," he says farther on, " but that the jasper of 

 Ascension may be viewed as a volcanic rock silicified, in precisely 

 the same sense as this term is applied to wood, when silicified." 



^ " Geological Observations on the Volcanic Islands visited during the voyage of 

 H. M. S. Beagle" (1844), p. 46. 



