PLATE xvu i. 



FIG. I. 

 PLAGIOCLASE-AUGITE-OLIVINE ROCK. GRANITIC TEXTURE. 



OLIVINE-GABBRO. LOCH CORUISK, SKYE. 



Magnified 85 diameters. The felspar is represented with crossed nicols : the augite 

 and olivine are drawn with ordinary light. 



The minerals represented are plagioclase (10), augite and diallage (7) and 

 olivine (1). 



The felspar occurs in the form of grains and aggregates of such grains. It shows 

 twinning on the albite and sometimes also on the pericline type. 



The pyroxene is partly in the condition of augite, and partly in that of diallage. 

 The two conditions are often illustrated in one and the same crystalline individual. The 

 fine striation indicative of diallage is due to the presence of minute rod-like or tabular 

 inclusions, arranged in planes parallel or approximately to the orthopinacoid. This can 

 be determined by an observation on the section of which a small portion only (7a) is included 

 within the field view at the top. This section shows three well marked cleavages in 

 addition to the plane of easy separation marked out by the inclusions. Two of these 

 are the prismatic cleavages, for they intersect at right angles, and the section shows in 

 convergent polarised light an optic axis on the margin of the field of view. The optic 

 axial plane (010) lies parallel with the third cleavage, and at right angles to the plane in 

 which the inclusions lie, which is thus determined to be the orthopinacoid (100). 



The olivine is traversed by cracks, along which iron oxides have been formed. 



The section from which this figure is taken is thicker than usual, so that the 

 felspar gives chromatic polarisation. 



FIG. II. 

 HORNBLENDIC GABBRO. 



CROUSA DOWN, NEAR COVERACK, CORNWALL. 

 Magnified 40 diameters. 



The minerals represented are uralitic (8a) and actinolitic hornblende (8b) and 

 plagioclase (10). 



The uralitic portion of the hornblende is fibrous, but the axes of the fibres are 

 not rigidly parallel with each other. The central portion of the figure is occupied by a 

 confused aggregate of fibres and grains, giving vivid chromatic polarisation under 

 crossed nicols. At the margin of this portion is a fringe of actinolitic hornblende. The 

 pleochroism of this hornblende is as follows : a = pale green, fi = pale brown or 

 brownish green, 7 = bluish green. The felspar which may be determined by 

 observations on cleavage flakes to be labradorite is very fresh. It frequently shows twinning 

 lamellae under crossed nicols, and the lamellae are often bent. The crystals are some- 

 times cracked. Here and there the felspar contains cloudy brownish patches. The 

 actinolitic fringe projects into the felspar substance, and detached needles of actinolite 

 occur abundantly as inclusions in the felspar. 



In other portions of the slide diallage may be seen partially metamorphosed to 

 uralite and actinolite. 



