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dodecahedra. Orthite (allanite) must also be mentioned as an accessory 

 constituent of granitic rocks. Andalusite occurs as an accessory constituent 

 of the Cheese- wring granite. It is often idiomorphic so far as the prismatic 

 zone is concerned. The optical characters of this mineral will be described 

 in the chapter on contact metamorphism. 



Cassiterite. This mineral occurs as an accessory constituent of granite 

 in the neighbourhood of tin veins. The crystals are usually short and 

 columnar. Cross sections are rectangular and extinguish when the cross 

 wires bisect the angles formed by the meeting of the faces of the prism. 

 They show in typical crystals the figure of a uniaxial mineral with positive 

 double refraction when viewed in convergent light. Sometimes, however 

 the black cross separates into hyperbolae as the stage is rotated. Longitu- 

 dinal sections are lath-shaped and give straight extinction. In thin section 

 the mineral may be colourless, yellow, brown or reddish brown, very 

 frequently an exquisite zonal structure may be observed. Refraction 

 and double-refraction are both very strong. 



Ground-miss of the traohytic rooks. The trachytic representatives of 

 the acid rocks are largely composed of a substance which cannot be resolved 

 into distinct minerals by the unaided eye. Sometimes the rock wholly 

 consists of such a substance ; as a rule, however, it contains porphyritic 

 crystals. Although not capable of resolving this substance the eye readily 

 distinguishes two distinct modifications the glassy and the felsitic. The 

 glassy substance may have a vitreous or resinous lustre the former 

 characteristic of obsidian, the latter of pitchstone. When examined under 

 the microscope the glass is usually seen to contain microlites, globulites, 

 rnargarites and trichites. It is as a rule devoid of double-refraction. 

 Nevertheless we sometimes find that the space surrounding the larger 

 crystalline constituents gives a reaction with crossed nicols indicative of a 

 state of strain. If the crystalline body be approximately circular in outline 

 in the section the space surrounding it will show four light quadrants 

 separated by the arms of a black cross. These arms will correspond in 

 direction with the principal planes of the nicols. In other words the space 

 surrounding the crystalline grain will behave as a spherulite yielding a 

 black cross with this difference, that the exterior will be ill-defined. 

 Perlitic structures are frequently present in the glassy rocks. 



The felsitic substance differs from glass in being dull and stony in 

 appearance. In chemical composition it agrees with many obsidians and 

 pitchstones except that it contains little or no water. It consists essentially 

 of silica, alumina and alkalies with small quantities of iron oxide. The 

 ratio of alumina to alkalies is essentially the same as in the alkali-felspars, 

 but the silica is in excess. The characters of this felsitic substance, or 

 felsite as it may be termed, have been made the subject o researches by 

 many observers and the most diverse opinions have been expressed as to 

 their true significance. In pre-microscopic days petrographers possessed 

 no methods by which the constituent parts of compact rocks could be 

 recognized. The introduction of the microscope has given us a large 

 amount of additional information, at the same time it has by no means 



