PLATE xvi. 



FIG. I. 



PLAGIOCLASE- ATJGITE-OLIVINE-MICA ROCK. 



BALLACHULISH, SCOTLAND. 



Magnified 10 diameters. Ordinary light. 



The minerals represented are plagioclase (10), augite (7), olivine (1), mica 

 (9), magnetite and green decomposition products forming a narrow zone round 

 each of the olivines. 



The plagioclase occurs in the form of extremely irregular grains and plates, and 

 sometimes also in forms which give lath-shaped sections. The boundaries of the 

 individual plagioclase grains cannot, however, be made out without the use of polarised 

 light. 



The augite shows a very pale greenish tinge, which is somewhat exaggerated in the 

 figure for the purpose of distinction. It contains inclusions of the brown mica, and 

 sometimes also of minute magnetite crystals. 



The mica is a rich brown variety. It occurs in irregular plates, which are often 

 penetrated by the felspar, and must, therefore, have been formed after the latter mineral. 



Olivine occurs in large grains. It appears grey when examined under a low 

 power, in consequence of numerous extremely minute inclusions. When examined 

 with a high power these inclusions are often seen to be aggregated in one or more sets 

 of parallel bands. Thus, in the large grain near the bottom of the figure to the right 

 two sets of bands, intersecting approximately at right angles, may be seen with a high 

 power. The olivine substance itself appears to be perfectly colourless. (See JUDD, 

 Q.J.G.S., 1885, p. 273.) The olivines are also traversed by irregular cracks, along 

 which magnetite has been formed. Each olivine grain when it comes in contact with 

 felspar is surrounded by a narrow fringe of a green substance, giving aggregate 

 polarisation. 



The order of separation of minerals in this rock appears to have been magnetite, 

 olivine, felspar, mica and augite. Original magnetite is, however, comparatively rare. 



FIG. II. 

 PLAGIOCLASE-OLIVINE-AUGITE ROCK. 



(OLIVINE-GABBRO.) 



Isle of Mull. Magnified 10 diameters. Ordinary light. 



The minerals represented are plagioclase (10), olivine (1), augite (7) and magnetite. 

 The felspar occurs mostly in the form of irregular grains. The augite is the pale 

 green variety. It occurs in small irregular plates which are interfered with by the 

 felspars. 



The olivine is extremely abundant, and presents characters which are somewhat 

 similar to those of the same mineral in Fig. I. The minute inclusions are, however, 

 less numerous. 



