METHODS OF PETROGRAPHICAL RESEARCH. 215 



Schuster (1SS0) leaves little to be desired. Cleavage- 

 flakes parallel to the basal or to the brachypinacoidal plane 

 are examined, and the extinction-angle determined with 

 reference to the trace of the other cleavage. Schuster 

 showed that the angles so obtained, with a proper con- 

 vention of positive and negative directions, are perfectly 

 characteristic of the several types of felspars. In the more 

 general case, however, the desideratum is some means of 

 identifying the different felspars as seen in thin slices of 

 rocks, and often in the form of minute microlites. Fouque 

 and Michel Levy attacked this problem with some success 

 in their Mineralogie micrographique (1879). Their method 

 was, in the first place, to calculate from known data, and to 

 plot by a curve, the variation in the extinction-angle for all 

 sections belonging to a determinate zone. For ordinary 

 plagioclase crystals twinned on the albite law the convenient 

 zone is that perpendicular to the brachypinacoid, and there- 

 fore to the twin-lamellae. Sections lying in, or nearly in, 

 this zone are easily recognised in a slice by the symmetrical 

 extinction of the two sets of lamellae. The extinction- 

 angles for such selected sections may vary from zero to a 

 definite maximum characteristic of the particular felspar 

 examined, and measurements of three or four 'suitable 

 sections will give the maximum with fair approximation. 

 With a sufficient number of crystals cut by^the slice, this 

 test is enough to distinguish most of the plagioclase fel- 

 spars ; but, owing to the fact that it is usually impossible 

 to discriminate between positive and negative extinction- 

 angles, an ambiguity arises between the albites and certain 

 varieties of andesine. For microlites, which are elongated 

 parallel to the intersection of the two principal cleavages, 

 the zone containing those cleavage-planes was investigated, 

 but here also an ambiguity was shown to occur in the case 

 ol certain felspars. Despite these drawbacks the method 

 can very often be applied with confidence. In 1888, and 

 again in 1894, Michel Levy (15), (16) rectified by the use 

 of new data some of the maxima oriven in the former work. 

 He also showed how the position of equal illumination of the 

 twin-lamellae affords a delicate test whether a given crystal is 



