1080 THE MICKOSCOPE IN GEOLOGICAL INVESTIGATION 



results are generally obtained by crushing up a small fragment of 

 the rock itself and mounting a few selected flakes, which can readily 

 be arranged for examination. Indeed, the study of a little powdered 

 rock is often valuable as an adjunct to that of a section, and when we 

 have some special purpose in view, or specimens do not promise to be 

 interesting, it may even obviate the necessity of cutting slices. 



The researches of the late Max Schuster have established the im- 

 portant fact that in the normal plagioclase felspars, which may be 

 considered as isomorphous mixtures of albite (Na 2 (Al 2 )Si 6 O 16 ) and 

 anorthite (Ca(Al 2 )Si 2 O 8 ), the optical and chemical characters stand 

 in the closest possible relations to each other. Hence, given the 

 extinction angle on a known surface, the chemical constitution is 

 known and, roughly speaking, the specific gravity. 



Another optical test of importance is the refractive index of a 

 mineral. The methods of measuring this are described in most of 

 the larger text-books, but much often enough for all practical pur- 

 poses can be done in a rough and ready way. For instance, 

 minerals with a high refractive index, such as diamond, garnet, 

 zircon, appear to stand out conspicuously on the slide. When they 

 occur in sand or the powder of a rock this is even more marked, and 

 internal reflection due to the large critical angle gives to the grain 

 a strong dark outline. Again, if a mineral with a high refractive 

 index be in apposition (as in a slice from a rock) with another having 

 a lower one, or with Canada balsam, and a quarter-inch objective be 

 used (with a plane reflector) and focussed on the top of the first 

 mineral, a thin bright line is seen just within its edge ; but when 

 the focus is changed to the bottom, this appears without the edge. 



The importance of pleochroism has been already mentioned. It 

 is not seen in colourless minerals, or in slices so cut as to be isotropic 

 in the plane at right angles to the path of the transmitted beam. 

 In augite it is generally weak, though visible in some green 

 varieties ; but in hornblende strong, especially in certain varieties. 

 Glaucophane exhibits a violet blue and a reddish purple ; riebeckite 

 turns almost black ; biotite, chlorite, amblystegite, and tourmaline 

 show it well, but in iolite it can be seen only in thick slices. The 

 student should note the results as the polarised beam vibrates parallel 

 with each axis of elasticity ; these facts, however, as a rule, are more 

 important to the petrographer than to the petrologist, and the latter 

 will not find it worth his while to spend time in determining them. 



The polarisation tints of a mineral, i.e. those seen with crossed 

 nicols, depend to some extent on the thickness of the slices, as has 

 been already stated, but they are often variable even in the same mineral . 

 Hence, though, as a rule, the student will find each species gives a 

 certain group of tints in the order of the chromatic scale, he must 

 be prepared for abnormalities. For instance, quartz, when it occurs 

 in a granite, usually gives high tints, but in a trachyte they are 

 rather low. At first the student must be cautious in drawing 

 inferences from polarisation tints, but after a certain amount of 

 practice he may do this with more confidence, though he will rely more 

 on the 'quality' than on the 'quantity' of the colour. For in- 

 stance, though both augite and olivine usually afford rich colours, an 



