120 Scientific Proceedings, Royal Dublin Society. 



olivine-basalt, and lience probably a greater rarity than the 

 well-known Lapis variolatus which strews the mountain-slopes 

 around the sources of the Durance. 



Note. — In connexion witli the above observations, I have examined the coarsely 

 spherulitic lava which forms an interesting feature of the summit of Aran Benllyn, 

 near Bala. This has no external resemblance to variolite, but proves to be a spherulitic 

 andesite similar to that of the Vashegy mountain in Hungary, but of a more liighly 

 silicated type. The glass between the microlites of the groundmass is now represented 

 by clear green products of decomposition, as in a tachylyte already described from 

 County Down. [Quart. Jottrn. Geol. Soc. xliv., p. 305.) 



EXPLANATION OF PLATE X. 



Fig, 1. — Yariolite of Ceryg Gwlaclys, showing the relations of the 



spherulites and the altered glassy matrix, . . x 9 



Fig. 2. — Silicified compact Diabase, retaining the original radial 

 fibrous structure, 7 mm. from the green variolitic 

 crust. An altered crystal of olivine, with separa- 

 tion of magnetite, is shown, . . . . x 30 



Fig. 3. — Corroded crystal of Olivine, of elongated form, from the 

 variolite of Ceryg Gwladys. The original is 3 mm. 

 in length, and is altered into pale green products, 

 in which yellow epidote has developed, . . x 25 



Fig. 4. — Crystals of Olivine from various sections of the variolite 

 of Ceryg Gwladys, showing corrosion and the de- 

 velopment of epidote, . . . . . . x 30 



Fig. 5. — Yeins of calcite, chalcedony, and quartz, traversing the 

 compact diabase, showing how brecciation might 

 produce the fragmental calcareous rock which is 

 found associated with it, . . . . . x 9 



