514 Scientific Proceedings, Royal Dublin Society. 



selvage, the glass clearly predominates. The spherulites are here 

 far more scattered, though often arranged in delicate bands by 

 flow parallel to the walls of the dyke ; and their average diameter 

 is only 1'25 mm. wide. Finally, the spherulites become reduced to 

 about -2 mm. in diameter, and there is a selvage, 5 mm. wide, 

 minutely jointed like that of the Basalt-glass of Portree,^ almost 

 free from products of crystallization, and even retaining its 

 original vitreous lustre. If any link was yet wanting between 

 variolite and tachylyte, the dyke of Annalong completely fills the 

 gap. 



The delicately banded rock, about 10 cm. from the selvage, 

 has a specific gravity of 2-68 to 2*72. The central mass probably 

 differs little from this at any point, a large specimen selected 

 giving 2"70. A good deal of carbonate of lime has been formed 

 during alteration, as is shown by the ready effervescence of the 

 rock-surface when touched with acid. The variolite of Anglesey 

 has a specific gravity of 271 ; and that of Mont-Genevre, very 

 possibly through the development of a larger quantity of epidote, 

 is as high as 2-91.^ 



Microscopic examination of the rock of Annalong shows that 

 it contains porphyritic crystals of plagioclase, which are too rare 

 in the sections examined to allow of specific determination. They 

 have clearly been formed during an earlier period of consolida- 

 tion, and are greatly corroded by the matrix. Their outer layers 

 have been penetrated to such an extent by filaments of glass as 

 to resemble the tubulated walls of foraminifera. One large decom- 

 posed pyroxene has also been observed. There is every reason to 

 believe that olivine also may be found, as at Anglesey, in some 

 portion of the dyke. The neighbouring dykes consist of olivine- 

 basalt, and the three that have been examined in section show 

 glassy selvages, one being truly variolitic for a distance of 15 

 mm. from each margin. 



The actual edge of the Annalong variolite, in contact with the 

 sedimentary rocks, shows under the microscope as a brown 



1 Judd and Cole, " Basalt- Glass of Western Isles of Scotland," Quart. Journ. Geol. 

 Soc, vol. xxxix., p. 451. 



2 Delesse similarly records 2"896 for the variolite of the Durance (Comptes Eendus, 

 t. XXX., p. 741). 



