116 Scientific Proceedings^ Royal Dublin Society. 



An apt parallel is drawn between this instance and tlie replace- 

 ment of trachytic rocks by silica in Hungary ; while the source of 

 the silica in Ascension is traced to the adjacent altered trachytes. 



With this high authority for the view above put forward in 

 explanation of the phenomena of Ceryg Grwiadys, we may proceed 

 to examine the microscopic evidence as to the process of replace- 

 ment and the condition of the variolitic crusts. 



Prof. Blake has kindly lent me a thin section of the variolite 

 as described and figured in the Report to the British Association. 

 In this an amygdaloidal structure happens to be prominent, which 

 is more rarely seen in the eight other sections which I have had 

 prepared from various parts of the rock-mass. Hence Prof. Blake 

 was led to refer the varioles visible on the surface of the rock to 

 the infilled vesicles, instead of to the spherulitic aggregations. He 

 describes the latter, but was not in the possession of favourable 

 means of judging as to their characters and importance. There 

 is no doubt, however, that this Anglesey rock is a true variolite, 

 and is not one of the amygdaloidal diabases which have been so 

 often confused with the spherulitic types. Prof. Blake mentions 

 " areas now composed of an aggregate of epidote crystals, and 

 having the external form of orthoclase crystals. There are also a 

 few small patches of augite." Certainly, there is an irregular 

 crystal in the section which he has lent me which may be a broken 

 pyroxene ; but, with this exception, as will be seen presently, I 

 refer all the porphyritic crystals that occur in the series of slides 

 to the one mineral, olivine. 



The general structure of the variolite precisely resembles that 

 of the types from the source of the Durance. We have the same 

 grouping of the somewhat irregularly bounded spherulites, first in 

 twos and threes, then in larger groups, until they are crowded 

 together to the exclusion of the glassy matrix (Plate X., fig. 1). 

 Hence in many parts we have merely the fan-like radial bunches 

 (fig. 2), composed of imperfect felspars, which are characteristic 

 of the rapidly cooled selvages of many diabases.^ But in this rock 

 of Oeryg Gwladys we have also the far rarer case, where a true glass, 

 a coarsely spherulitic tachylyte, has remained upon the surfaces of 



^ See, in addition to the conipacter rocks of Mont Genevre, the " Sordawalite, " 

 studied by Loewinson-Lessing (Tscheini. Mittheil. Bd. ix., 1887, p- 61). 



