OF THE WESTERN ISLES OF SCOTLAND. 365 



I. Analysis of glass in labradorite-andesite from Bealach a' Mhiiim, 

 Skye, by S. Parrish. 



II. Analysis of second specimen of the same rock by H. J. 

 Taylor. 



III. Mean of these two analyses. 



It appears from these analyses that this glass is richer in silica 

 than the andesite (a labradorite-andesite) in which it occurs. 



Studied microscopically, this vitreous rock is found to consist of a 

 glassy base, less black and opaque than ordinary tachylyte, through 

 which porphyritic crystals of plagioclase (labradorite) are somewhat 

 sparsely scattered. The glass often contains incipient and some- 

 times well-formed spherulites, and the arrangement of these and of 

 darker streaks of glassy material give it a marked fluidal structure. 

 The felspar phenorysts are remarkable for the amount of corrosions 

 by the fluid magma which they have undergone, and spherulitic 

 fringes have often been developed all round the edges of the 

 crystals. Sometimes the incipient spherulites are seen to yield to 

 weathering influence much more readily than the enclosing glass, 

 and a peculiar banded appearance then becomes very conspicuous 

 on weathered surfaces (see Plate XIY. fig. 5). 



The Augite-diorites. — In 1866 Zirkel* proposed the use of this 

 term, and in 1877 Strong described an important class of rock of 

 this type as occurring in Minnesota f. Mr. Cole has also strongly 

 advocated the use of this term J, ^^hich has, moreover, been adopted 

 by Eosenbusch, in the last edition of his ' Massige Gesteine.' 



The type is beautifully exemplified, especially in Arduaraurchan, 

 where great mountain-masses, like that of Meal nan Con, are made 

 up of it. The augite often exhibits, partially or throughout, the dial- 

 lagic striation, and the rocks diff'er from the gabbros only in the 

 absence of olivine and magnetite. Every gradation can be followed 

 from rocks with a glassy magma, though various granophyric types, 

 into a perfectly holocystalline rock. 



Quartz-augite-d writes occur at several points, as in the great sheets 

 under Beinn More. They contain both rhombic and monoclinic 

 pyroxene, and also quartz, both primary and secondary. 



As it has been asserted that the " felstones " or propylites of the 

 Western Isles of Scotland are really nothing more than basalts 

 altered by contact-metamorphism, I may point out that, asso- 

 ciated with the andesites and more acid rocks, are a few ophitic- 

 olivine-basalts, which have been subjected to the same kind of 

 modification as the propylites. These are found to exhibit cha- 

 racters very strongly contrasted to those of the rocks in question. 

 In all, or nearly all, these cases the distinctive characters of the 

 basalts can still be clearly recognized, namely, the olivine grains, 

 reduced to pseudomorphs, and the ophitic structure, traces of which 

 can be detected even when both the felspar and the pyroxenes have 

 undergone the most profound change. 



* Lelirbuch der Petrograplue, vol. ii. p. 7. 



t NeucR Jahrb. fiir Min. &c. 1S77. 



I Geol. Mag. dec. iii. vol. iii. (188G) p. 225. 



