350 Becker — Determination of Plagioclase Feldspars. 



such cases will arrange themselves. He has also plotted the 

 extinctions for this particular zone, and shows that when a 

 compound albite-Oarlsbad twin is cut in this zone it can in 

 almost all cases at once be referred to its proper species and its 

 proper orientation. 



By some mischance errors have crept into this diagram of 

 the extinctions of the feldspars in the zone of symmetry, and 

 1 have taken the liberty of replotting it (on Plate III) from 

 the stereographic projections. I have also added plots of the 

 extinctions at 10° from the zone of symmetry in either direc- 

 tion. In a thin section of a rock the chances are infinitely 

 against any feldspar being cut with mathematical precision in 

 the zone of symmetry. Hence the question at once arises how 

 the extinctions will vary in case the plane of symmetry is 

 slightly inclined. These supplementary diagrams give this 

 information at a glance and assure the observer whether or no 

 the variation of the orientation from the position of exact 

 symmetry precludes precise determination. 



In a very great number of cases feldspars exhibit traces of 

 zonal structure due to gradual variation in the composition of the 

 successively deposited layers of the crystal. Mr. Michel-Levy 

 has shown in his second fascicle that if one regards the several 

 plagioclases as mere mixtures of albite and anorthite, instead of 

 as independent species, the position of equal zonal illumina- 

 tion is absolutely characteristic of the orientation of a feldspar 

 in the zone of symmetry. There is only one angle in each 

 quadrant at which equal zonal illumination occurs. This 

 method can be used, for example, in determining when a crys- 

 tal is cut so nearly perpendicular to the prismatic axis that the 

 extinction of Carlsbad lamellae should be taken upon opposite 

 sides of the cross-hair. In that case the angle of equal zonal 

 illumination cannot exceed 11°. The angle of equal illumina- 

 tion as a rule is less sharply determinable than, for example, 

 that of equal illumination of albite lamellse. 



In his second fascicle Mr. Michel-Levy regards all the plagio- 

 clases as mere mixtures of albite and anorthite, and defines the 

 special occurrences in percentages. There is, of course, a very 

 great amount of evidence for this position, and, so far as the 

 needs of the working lithologist are concerned, it is probably 

 without sensible error. At the same time it may be remarked 

 that were there no dissipation of energy accompanying the 

 union of the albite and anorthite they would not tend to 

 unite. The liberation of energy corresponding to their actual 

 tendency to union must, one would think, be accompanied by 

 some modification in physical qualities ; but this change, so 

 far as known, is negligibly small. 



Mr. Michel-Levy's new attitude toward the feldspars involves 



