1078 THE MICROSCOPE IN GEOLOGICAL INVESTIGATION 



The optical methods now in use enable the petrologist to determine 

 the constituents of rock-masses with great success. The colour 

 of the mineral in transmitted light, the crystallographic outlines, 

 the direction of the cleavage planes, the polarisation tints, the posi- 

 tion of the axes of elasticity, as also of the optical axes, all these, 

 with other minor properties, render his determinations of real value. 

 In certain cases pleochroism is a valuable test ; this is well deve- 

 loped in such minerals as hornblende, biotite, tourmaline, etc. 



Yery important service has been rendered by the microscope in 

 the study of the phenomena known as optical anomalies. There 

 exist a large number of minerals which show in thin sections optical 

 properties which do not agree with those of the crystal system to 

 which they belong. Experiment has proved that compression, 

 strain, or other mechanical distortion, may cause amorphous bodies, 

 like glass, and crystals belonging to the regular system to become 

 double-refracting, and a uniaxial crystal becomes biaxial by the appli- 

 cation of pressure at right angles to its optical axis. 



Mention may well be made here of 

 the anomalies presented by the mineral 

 leucite, which is a most important con- 

 stituent of the lavas of Vesuvius and 

 the neighbourhood of Rome. It crystal- 

 lises apparently in icositetrahedra (fig. 

 809), and thus to belong to the regular 

 system it should remain dark under 

 crossed nicols, that is, be isotropic. The 

 small crystals certainly behave in this 

 manner, but the large ones display more 

 or less double refraction with decided 

 FIG. 809. Leucite showing twin- traces of twin-lamellae (fig. 809). This 



striation under crossed nicols. anoma ly was f or a lon ~ time inexplicable, 

 (After Zirkel.) , .,, T ^/ . , , , & 



till Klein showed l that such crystals 



revert when heated to 500 C. to a condition of perfect isotropv. 

 which property they again lose upon becoming cool. The conclusion 

 to be drawn from his classical investigation is that the leucite 

 originally crystallised in the regular system and that its present 

 optical condition is owing to molecular change due to strains set up 

 as the temperature falls during and after solidification. It is 

 worthy of notice that MM. Fouque and Michel Levy have syn- 

 thetically produced a leucite rock, the leucites of which possessed 

 the optical anomalies described above. 



The relation between optical characters and chemical constitu- 

 tion has received some degree of attention, and in the case of the 

 felspar group has been accurately determined . Only the ' quantitative ' 

 portion of the subject can be dealt with here, and we must abstain 

 from the discussion of those minerals whose microscopical appearance 

 leads the trained petrologist to draw qualitative conclusions. By 

 employing convergent light, a slice of a mineral, cut in the right 

 direction, can be examined and an 'optical picture' obtained. 



1 For a description of the so-called ' Erhitzungs-Mikroskop,' see Groth's Pliysi- 

 kalische Krystallographie, Leipzig, 1885, p. 631. 



