OF THE WESTERN ISLES OP SCOTLAND. 361 



Quartz-cUorites have a tolerably wide distribution in Mull and 

 some of the other centres of eruption ; the rocks in it, frequently 

 exhibited granophyric structures and other peculiarities, recalling 

 in the most striking manner the quartz-diorite of Doire na Each 

 and other bosses in Arran so well described by Professor Zirkel *. 



B. Pijroxene-andesites and Pyroccene-diorites. 



These rocks contain as phenocrysts (or minerals of the first con- 

 solidation) felspars which are always plagioclastic, and which, by 

 their characteristic extinctions, their specific gravity, and their flame- 

 reactions, are shown to belong to labradorite or to a variety between 

 labradorite and anorthite. These porphyritic crystals are note- 

 worthy as very constantly displaying a zoned structure, and are 

 usually full of glass and stone-enclosures, for the most part arranged 

 parallel to the sides of the crystals. The crystals often exhibit the 

 evidences of growth after the consolidation of the rock, a phe- 

 nomenon which has been already described. Sometimes in addition 

 to porphyritically developed felspars we find large crystals of 

 augite, belonging to a variety very rich in magnesia and iron, and 

 often exhibiting the structure to be hereafter described as the 

 " pseudo-diallagic ; " enstatite not unfrequently accompanies the 

 augite, a ferriferous variety — between bronzite and hypersthene — 

 being the most common form of the mineral. Olivine is either 

 entirely absent or is so rare in these rocks that it must be regarded 

 as an accessory or accidental constituent only. Magnetite, however, 

 is always present, though in very varying quantities. 



The minerals of the ground-mass, or those of the second period 

 of consolidation, consist of felspars (usually showing lath-shaped 

 sections and more or less lamellar twinning), which by their extinc- 

 tions are referable to oligoclasc, but may sometimes be orthoclase ; 

 intercrystallized with the rod-like felspars is a pale brown variety 

 of augite, usually occurring in more or less rounded granules, and 

 many opaque magnetite grains. 



The glass, which sometimes is almost absent in these rocks, and 

 at other times forms the greater part of their mass, is usually full of 

 crystallites, and in the arrangement of bands and flecks of different 

 colours, or the distribution of the crystallites, shows striking evidence 

 of flow-structure. This is especially manifest in the varieties 

 which contain porphyritic constituents and much glass. Skeleton 

 crystals and rods of magnetite are very abundant in these glassy 

 bases of the audesite-rocks. 



The pyroxene-andcsites of the district fall naturally into two 

 groups, between which, however, many connecting-links may be 

 found. 



Those rocks in which the quantity of glassy base is reduced to a 

 minimum, and which consist largely of the minerals of the second 

 period of consolidation, with or without porphyritic constituents, 

 undoubtedly approximate to the basalts. liut their real analogies, 



^ Zeitschr. d. deutsch geol. Gesellscb, vol. xxiii. (1871) p. 30. 



