OCCURRENCES OF TACHYLYTE. 301 



colour is a rich blue-black. Its high specific gravity, which reaches 

 2*83, may be explained by the crowding of spherulites in mnny 

 parts and the consequent approach to a crystalline condition. The 

 rock, however, that forms the centre of the intrusive sheet has a 

 specihc gravity of only 2*79, as determined both by the chemical 

 balance and by Attwood's instrument on a large and representative 

 specimen. This anomaly parallels the examples cited by Delesse *, 

 who himself recognized how the density of different portions of an 

 intrusive mass might be affected by decomposition at the centre 

 rather than at the margins t. 



The hardness of the glass of Ardtun is 6, and its fusibility equals 

 2"5 of von Kobell's scale, the product of fusion being a brown glass 

 full of bubbles, which is blown out almost to a pumice when treated 

 in the blast of a Herapath blowpipe. The powder of the rock is 

 not attracted even by a powerful bar-magnet. 



Under the microscope this tachylyte is found to repeat in the 

 basic series the transition from giassj^ to completely spherulitic forms 

 which is so familiar among acid lavas. Near the surfaces of junction 

 the glass, rich orange-brown when thinly ground, is full of crystal- 

 dust and shadowy aggregations. A few minute amygdaloidal vesicles, 

 elliptical in section, are scattered here and there, the glass being 

 lightest round them, this zone of different hue being very likely due 

 to alteration spreading inwards. During the infilling of such 

 cavities, the surrounding glass may often undergo a change, this 

 being notably the case in the tuft' of Aci Reale, Sicily, where every 

 vesicle in the fragments of basalt-glass is bordered by a ring of 

 brown palagonite. By gradual extension these rings unite, and 

 effect the alteration of whole areas. 



The Ardtun glass contains spherulites in all stages of development ; 

 and the brown globulitic matter of which they are composed is more 

 and more condensed towards their centres, where they become 

 practically opaque. A layer of spherules has formed in places on 

 the very surface of the intrusive sheet, and the condensation of 

 material towards the flattened side of these has given rise to a 

 dark band along the plane of contact. A similar instance of the 

 separating-out of materials on the plane of junction is described by 

 Mr. itutley %, who has kindly allowed me the use of his original 

 slides. In both of these cases, the artificial and the natural, I 

 take it that no very sudden chilling has occurred, and in the sub- 

 sequent gradual cooling the crystallites have utilized the surface 

 presented to them as a basis of aggregation, just as they cluster 

 round a porphyritic crystal or any similar inclusion (PI. XI. fig. 1). 



Eurther from and parallel to the edge of the intrusive sheet, bands 

 of brown spherulites traverse the darker glass, which is here more 

 opaque through accumulation of minutely separated matter ; and 

 finally the glass is practically eliminated, the spherulites assuming 

 polygonal outlines as they come in contact with one another 



* Metamorphisme des Eoches, pp. 40S-407. 



t Ibid. p. 406. 



X Proc. Roy. Soc. vol. xl. (1886), p. 438. 



