118 Scientific Proceedings, Royal Dublin Society. 



able lavas of Hawaii. The glassy crusts of Ceryg Gwladys, 

 equally with those of the Western Alps, probably point to con- 

 ditions of high temperature and complete fusion such as are not 

 often met with in volcanic centres. The presence of abundant 

 traces of olivine in the Anglesey rock shows that variolite may be 

 produced from basalts rich in olivine as well as from the augite- 

 andesites, and that tlie structure is more dependent upon conditions 

 of original liquidity and of cooling than upon peculiarities of 

 chemical composition. 



The replacement of olivine or monticellite to so great an extent 

 by epidote is certainly unusual, and I find no record of such a case 

 even in Hintze's treatise on Mineralogy. But the epidote, which 

 is also so common throughout the ground-mass, has probably been 

 derived from materials external to the porphyritic olivines, which 

 were originally poor in iron, produced but little magnetite, and 

 so gave way entirely during the complete permeation of the 

 mass. 



The dusky gray spherulites are characterized by the well- 

 known rifts or " pseudo-crystallites," which also invade the ground- 

 mass when this is full of crystallizing material. The glass left by 

 the concentration of this material into spherulites seems to have 

 been fairly free from microlites, and is now altered into green 

 isotropic areas and minute granular epidote (fig. 1). The spheru- 

 lites, with more or less distinct radial structure, have included for 

 the most part the corroded and porphyritic crystals; brown " crystal- 

 dust" continued to gather round them from the matrix, just as 

 cloudy masses are seen round the well-formed spherulites in the 

 obsidian of Yulcano ; between these cloudy aggregations the true 

 tachylytic glass finally remained. 



The minute fan-structure, and the still more compact and 

 crystalline microlitic " felt," resulting from the copious develop- 

 ment of microlites in the more slowly consolidating interior of the 

 rock, have been beautifully preserved in those parts wliich have 

 been invaded by tlie jasper (fig. 2j. Mr. Darwin's suggestion thus 

 receives am^ple verification. The veins of calcite traverse the dull 

 purple siliceous areas, and hence have arisen subsequently to the 

 pseudomorphic action. 



Where the variolite has suffered from other causes and has 

 become fissile and even slaty in the mass, sections show consider- 



