GABBROS ETC. IN SCOTLAND AND IRELAND. 91 



the diversity of their orientation, as is shown when they are examined 

 by the aid of the polariscope. Some of the magnetite is probably 

 original, but a considerable part of it is clearly secondary, and formed 

 by the alteration of the olivine. The augite in places shows the com- 

 mencement of schillerization, and tends to pass into diallage. Por- 

 tions of this rock become porphyritic by the additon of large, zoned 

 crystals of felspar (see p. 68). 



Fig. 3. Semi-ophitic structure in basalt from Portree, Isle of Skye. This is a 

 good example of a type very characteristic of the coarser-grained 

 basalts of the district. The amount of unindividualized glass is very 

 small, so that the rock is almost a dolerite, according to the definition 

 which we have adopted. The lath-shaped felspars are narrower and 

 less clearly defined in outline than in Fig. 1. The augite, though usu- 

 allj' behaving in the ophitic manner, sometimes breaks up and becomes 

 granuUtic ; and this is more conspicuously the case with the olivine 

 and magnetite. The rock contains large porphyritic crystals of olivine. 

 This type, so common among the coarser basalts of the Western Isles 

 of Scotland and the North of Ireland, is admu'ably represented in 

 Iceland (see E. Breon's 'Iceland,' plate li. fig. 2). 



Fig. 4. Semi granulitic structure in basalt from Ru Geur, Isle of Skye. This 

 rock, like the last, contains only a trace of a' vitreous ground-mass, 

 and may, like it, be regarded as one of the links between the dolerites 

 and the basalts. The felspar forms narrow and often ill-defined lath- 

 shaped crystals ; the augite is usually granular, but in places shows 

 an approximation to the ophitic type, the orientation of adjacent 

 grains which enclose the felspar crystals being not unfrequently the 

 same. The rock contains porphyritic crystals of both olivine and 

 felspar. This type, like the last, is very frequently represented among 

 the coarse-grained basalts forming the more massive lava-currents of 

 the district. The two types pass into one another by insensible gra- 

 dations. 



Fig. 6. Micro-ophitic structure in the basalt of Dumfrin, near Tobermory, Isle 

 of Mull. In the section of this lava we find the j^brown augite deve- 

 loped around the small lath-shaped microlites of felspar so as to produce 

 a very perfect ophitic structure on a very minute scale (compare with 

 fig. 1, bearing in mind that fig. 5 is magnified three times as much as 

 fig. 1). The rock contains grains of olivine, which is converted into 

 serpentine, and is rendered porphyritic by large crystals of plagio- 

 clase felspar. There is a small quantity of glassy ground-mass in the 

 rock. So far as my experience goes, this micro-ophitic structure is 

 only of rare occurrence. 



Fig. 6. Micro-granulitic structure in a basalt from Ardnamurchan. In this 

 rock the granulitic structure is exhibited on a very minute scale (com- 

 pare with figure 2, bearing in mind the difference of amplification). 

 In a plexus of short felspar crystals we find scattered rounded grains 

 of pyroxene and olivine, the diverse orientation of these grains ap- 

 pearing very distinctly when the section is viewed by polarized light. 

 The rock is not porphyritic, and the particular type of structure 

 appears to be somewhat rare. 



Fig. 7. Structure seen in basaltic dyke in the Isle of Scalpa, near Skye. The 

 felspar and augite appear as somewhat ill-defined microlites, the 

 latter exhibiting a fibrous appearance. The whole of the rather 

 abundant glassy base is filled with rods and skeleton-crystals of mag- 

 netite, which sometinies start from the fiides of the augite microlites. 

 The imperfect crystallization of the several minerals is very well seen 

 when the section is examined by polarized light. 



Fig. 8. Structiu*e seen in the basalt of a lava-current exposed at the western 

 foot of Beinn Inivaig, Isle of Skye. In this section we have the gra- 

 nulitic structure well exemplified, but the granular microlites of 

 augite are smaller and less perfectly defined than in figs. 4 and 6 ; 

 and the surrounding plexus of felspar-microlites is less distinctly in- 

 dividualized, the quantity of glass in the ground-mass being much 



