GAB BROS ETC. IN SCOTLAND AND IRELAND. 93 



felspar is represented by very ill-defined lath-shaped microlites, and 

 the augite by granular particles of which the outline is also ill-defined, 

 with others in which the polarization is very obscure — in fact they 

 should be called crystallites rather than microlites ; naore or less 

 rounded grains of magnetite, with *' magnetite-dust" and magnetite 

 globulites, are also scattered through the glass. 



Fig. 7. Magma-basalt from a dyke at Gribun, Isle of Mull. This differs from 

 fig. 5 in the absence of any distinct crystalline forms. In the mass 

 of glass the felspars are probably represented by ci-ystallites in the 

 form of long rods often bifurcating at the ends, and it is doubtful if 

 any other mineral except magnetite has begun to separate from the 

 glass. The magnetite forms an intricate network of rods and skeleton- 

 crystals, which are sometimes seen to spring from the sides of the 

 embryo-crystals (crystallites) of felspar. 



Fig. 8. Magma-basalt from a lava-stream at Bloodstone Hill, Isle of Eum. 

 Through the abundant pale brownish glass of this rock we find 

 scattered groups of short crystallites (belonites) of felspai', granular 

 crystallites of augite, and grains of magnetite, these embryo -crystals 

 being usually collected into groups. The contrast between the rocks 

 represented in figs. 7 and 8 is very striking, and is evidently the 

 result of differences of condition during the consolidation of the two 

 rocks. 



[The completely vitreous basalt-glass or tachylyte has already been 

 sufficiently illustrated (see Quart. Journ. Greol. Soc. vol. xxxix. pi. xiv. 

 figs, 1, 2, 3, 4, 5). In these the sections are represented as seen with 

 a still higher power (500 diameters) than we have employed for the 

 magma-basalts. These tachylytes are from the edges of the dykes, and 

 in all of them except fig. 1, rods and skeleton-crystals of magnetite are 

 thickly scattered through the glassy base. Sometimes these rods are 

 seen to be lines of globulites (" margarites" of Vogelsang). The 

 cloudy granular condition of the magnetite in fig. 1 may be the result 

 of a certain amount of friction between the cooling glass and the walls 

 of the dyke.] 



Plate VII. 



Fig. 1. Crystal of felspar from the porphyritic gabbro of Ardnamurchan. 

 Natural size. The porphyritic crystals of felspar in this rock are 

 from 1 to 2 inches in length. In the example figured, it is seen that 

 the crystal, subsequently to its formation, has yielded to mechanical 

 forces, cracks being formed, generally along cleavage-planes, but some- 

 times passing irregularly through the crystal. These cracks have been 

 injected with material which has become perfectly crystallized ^as 

 augite. The whole crystal with its enclosures has undergone some 

 alteration, numerous fissures and enclosures being produced in the 

 felspar substance, while the augite has in places been darkened by the 

 separation of magnetite. 



Fig. 2, Porphyritic crystal of felspar in the granulitic dolerite of Ru Geur. 

 Magnified 75 diameters. This crystal, besides being very much broken 

 or eroded along its edges, has yielded along the planes of most easy 

 cleavage, and permitted the infiltration of augitic material, which has 

 crystallized. The contrast between this infiltrated augitic material 

 and the granulated augite in the base of the rock is very marked. Both 

 this and the last figure afford striking illustrations of the tendency of 

 the felspar in these rocks to crystallize before the augite. 



Fig. 3. Glomero-porphyritic structure in the dolerite of Fair Head, Co. Antrim, 

 Ireland. Magnified 2^ diameters. The ground-mass of tliis rock is 

 an ophitic mass, consisting of brown augite and dark-coloured olivine, 

 both enclosing lath-shaped crystals of felspar in great profusion. 

 Scattered through this base, we find aggregates of crystals of olivine 

 and anorthite ; in these aggregates the grains of olivine are generally 

 enclosed by a mass of the broad anorthite ci'ystals, so that the 

 relations of these two minerals is the same as is seen in troctolite 



