144 THE INCLUSIONS OF THE VOLCANIC ROCKS 



VI. QUARTZ-BEARING INCLUSIONS 



Quartz-Pyroxene Inclusions in the Trachyte of Mount Cis 



These are small, rounded, compact, greyish white inclusions, at first sight similar 

 to some of the microsanidinites, but distinguished from them on close examination 

 by the presence of small grains of quartz. With them must be included a number of 

 very vesicular brown inclusions, since in some cases the latter contain a kernel of the 

 former. 



Under the microscope the first type is seen to consist mainly of numerous large grains 

 of quartz separated by smaller grains of the same mineral and of a pale yellow- green 

 augite (Fig. 3, PI. III). Occasionally the pyroxene has a sieve- like or poikilitic develop- 

 ment, enclosing grains of the other minerals indifferently. In some specimens a turbid 

 plagioclase ( labradorite) also occurs in large grains, but never in amount equal to the 

 quartz. Wollastonite can be definitely identified in one section (E 14) in long prisms 

 enclosing small granules of augite, and is suspected in others. Magnetite appears 

 in most specimens in irregular blotches and small grains, and sphene is found in a few 

 in large rounded forms. Calcite appears in only two specimens. In all a greater or less 

 amount of a clear brown glass is present. 



Sometimes the structure of the inclusion remains unaltered right to its contact with the 

 host, but where the quartz grains abut against the trachytic ground-mass there is an 

 ingrowth of a green augite, near a?girine in its properties, from the host. 



This phenomenon is still better exemplified in cases where sporadic xenocrysts of 

 quartz have swum in the trachytic magma (Fig. 4, PI. III). At other times a 

 different form of contact is observed ; the inclusion becomes slaggy and vesicular 

 towards the edge, and in section is predominantly glassy with an occasional development 

 of large anorthoclase crystals besides the sharp augite prisms, some partially molten 

 quartz grains still remaining. 



There is little doubt that the wholly slaggy and vesicular inclusions represented 

 melted rocks of the same nature as those which gave rise to the quartz- pyroxene type. 

 The rocks are excessively fragile, and no satisfactory sections were obtained, but 

 examination of crushed fragments in oil shows that they consist chiefly of a clear brown 

 glass with a variable amount of quartz, augite, magnetite, and numerous prismatic 

 crystals of a mineral which could not be definitely determined. It has refractive indices 

 superior to those of wollastonite, which it resembles in its straight extinction and 

 moderate birefringence. It is biaxial with a low optic axial angle, is negative and has 

 positive elongation of the prisms. 



One specimen (E 17, T 5) merits separate description. It has a vesicular contact 

 zone with the host of about 1*5 cm. in width. The kernel (4 cm. in diameter) has 

 a thin white shell, but is much darker in the centre than is usual in this type of rock 

 (Fig. 5, PI. III). The contact zone is similar in general characters to the above described 

 rocks, containing the quartz, augite, and the undetermined mineral all lying in a brown 

 glass. The outer part of the kernel resembles the more compact inclusions. The dark 

 colour of the centre is due to the prevalence of finely distributed magnetite dust. Large 

 clear quartz grains are still present, but they are fewer and farther apart, and there 

 are in addition a few grains of an intermediate plagioclase. These minerals are all 

 enclosed in large interlocking plates of a mineral which resembles cordierite both in 

 habit and in the abundance of magnetite inclusions. It is biaxial with a moderate 

 axial angle, and optically negative and may thus be cordierite or a felspar of the ortho- 

 clase-anorthoclase series. No twinning was observed. On the margin between the 

 dark centre and the clearer exterior there are numerous granules of strongly pleochroic 

 hypersthene, often developed at the edge of large quartz grains. 



