OCCTJRBENCES OF TACHYLTTE. 303 



light entering it vibrates parallel to this same direction. It will 

 bo seen that the extinctions and pleochroisra of the browner rays, 

 as described above, are not opposed to the suggestion that they are 

 of pyroxenic character. 



Two and a half inches from its outer surface, the Ardtun basalt 

 has given rise to a multitude of interlacing rods, with here and there 

 traces of a radial arrangement. These rods, like those in the tachy- 

 lyte of Lamlash *, are themselves composed of individualized 

 granules grouped along definite lines. In this instance the constitu- 

 ent crystallites are of prismatic outline, and may be measured with 

 a high magnifying-power, being about '004 of a millimetre long. 

 As we trace this structure towards the interior of the mass, long 

 skeletons of colourless felspar are evolved, and round these are 

 clustered numberless little prisms and granules, the embryos of the 

 pyroxenes that abound at the centre of the sheet. 



A glassy selvage of even an inch in thickness would seem in itself 

 to indicate a basalt rich in silica and the alkalies, a rock, in fact, 

 on the threshold of the andesitic series f. A determination of the 

 chief chemical constituents of the Ardtun tachylyte has yielded me 

 the following result, the specimen selected being from the lower 

 selvage of the intrusive sheet : — 



Silica „ 53-03 



Alumina 20-09 



Ferric oxide 9-43 



Lime 6-05 



Magnesia 2*63 



Soda 4-52 



Potash 1-27 



Loss on ignition 2-64 



99-66 



The rock thus corresponds closely in composition with the basalt- 

 glass of the Beal in Skye t, and is another addition to the more 

 highly silicated and more aluminous examples, as distinct from the 

 lavas of Hawaii, which are far richer in magnesia and lime. 



The next occurrence of tachylyte to be described is at Kilmelfort, 

 on the Argjdl coast, where, in 1883, in a quarry just north of the 

 Cuilfail Hotel and opposite a tiny lake, I noticed a dyke, fringed 

 with glass, giving off a vein into the surrounding greyer rock. This 

 dyke was only from one to two feet across, and had merely a film of 

 glass upon its surfaces ; the vein connected with it, about two inches 

 in width, showed also a thin tachylytic selvage. In another part of 



*■ Quart. Journ. Greol. Soc. vol. xxxix. pi. xiv. fig. 2. 



t I am aware of the objection raised by Stecher to this inference (Tscher- 

 mak's Mittheilungen, 1887, p. 198) ; but almost all the tachylytic dykes de- 

 scribed from Scotland traverse basaltic masses, and the similarity of con- 

 ditions at the immediate edge has prompted the comparisons that have been 

 made. 



I Quart. Journ. Geol. Soc. vol. xxxix. p. 455. 



