Application of tJie Microscope to the Study of Rocks. 16 



shade oflf round the margin of the grain into a fringe of another 

 tint. In some rocks the grains of quartz, though apparently 

 homogeneous, split up by polarised light into a chromatic mosaic 

 — a phenomenon known as " aggregate polarisation." Each 

 grain is, in fact, made up of a number of distinct elements, or 

 crystalloids, with their optic axes variously orientated. 



The Mica of Granite is generally either colourless or dark 

 brown, the former being commonly refeiTed to the species 

 Muscovite and the latter to Biotite. In many granites both 

 micas coexist. The biotite, or dark iron-magnesian mica, is 

 strongly dichroic. To observe this dichroism plane polarised 

 light is transmitted through the section, and the analyser is 

 removed. On rotating either the section or the polariser below 

 the change of tint is very marked, varying between a pale 

 yellowish brown and a dark, almost black, colour. The mica 

 frequently exhibits a dappled or flecked appearance, and the 

 edges of sections across the basal plane seem to be ragged, while 

 the perfect basal cleavage is revealed by longitudinal striation. 

 The biotite frequently suffers alteration, and is converted more 

 or less completely into a green chloritic mineral, sometimes 

 called viridite. The determination of the exact species of mica 

 requires refined examination in convergent polarised light, and 

 it is therefore only in general terms that the two characteristic 

 micas of granite can be called muscovite and biotite. Hence 

 French petrographers, Hke MM. Fouque and Levy, generally 

 content themselves by referring to them simply as mica blanc and 

 77iica tioir. 



In addition to the felspars, the quartz and the micas, most 

 granites contain certain accessory minerals. Apatite, or phos- 

 phate of lime, is commonly present in small six-sided acicular 

 prisms, perfectly clear and colourless. Hornblende is common in 

 syenitic granites, and may sometimes be mistaken for biotite, 

 but is readily distinguished by its angle of extinction. It is 

 either green or brown in colour, and is strongly dichroic. The 

 simplest method of determining hornblende is to find a basal 

 section which exhibits the edges of the prismatic cleavage-planes ; 

 these edges form by their intersection lozenge-shaped patterns, 

 with the characteristic wide angle (rather more than 12-4°) of the 

 fundamental prism. Epidote occurs as a secondary ijroduct in 

 many granites, and may be distinguished by its clear pale 

 greyish yellow colour, and its high index of refraction, which 

 causes it to stand out prominently ia the field. Sometimes it 

 occiirs in curiously aggregated gum-like masses. Garnets are 

 frequent constituents of granite, and occur either as distinct 

 crystals, colourless or of pale pink tint, or as gummy blebs, 

 which seem to stand out m high relief. Since garnet crystallises 

 in the regular or cubic system it is isotropic; that is to say, the 



