380 The American Geologist. J""*^- ^^'^^• 
Methods for Dctennlning Feldspars by Ascertaining the 
Position of the Optic Axes. 
These methods are among the most practical and accurate. 
It is well known that in feldspars the position of the optic axes 
varies regularly in regard to the crystallographic boundaries 
according to the change of chemical composition, and it is only 
necessary, therefore, to define the relation of the axes to these 
boundaries to determine the feldspar. 
Michel Levy's Method. — This method, witli more or less 
variation, has been lately much used in this country. In its 
original form, as proposed by the great French mineralogist, 
it may be described briefly as follows : 
For this method feldspars must be twiimed. If twinned 
after the albite law, find a section in the zone of symmetry, or 
nearly so. This zone is characterized by having the extinction 
angle in the two individuals on both sides of the twinning plane 
the same. If the angle of the position of equal illumination 
of the two individuals with the twinning line is greater than 
that of the position of extinction with the same line, the feld- 
spar is more acid than oligoclase; if more basic, the reverse. 
If the feldspar is twinned after both albite and Carlsbad 
laws, discover this by bringing the albite lamellae in position 
of equal illumination — then the Carlsbad twinning will still 
show its differences of extinction. Find such a crystal in the 
zone of symmetry or nearly so, by the above method. Read 
the extinction angle on one side of the Carlsbad twinning plane 
and then on the other. Suppose these are 38° and 21°. Refer 
to Michel Levy's plates.* The vertical red line is the projection 
of the zone of symmetry. On the upper half of the projection 
the red figures equal + angles of extinction on the zone of 
symmetry ; on the lower half the black ones equal — angles on 
the same zone. In Carlsbad twinning the two individuals are 
turned on vertical twinning axes 180°. so that the negative 
parts of one individual become side by side with the positive 
parts of another. Therefore, the two p.ngles read on the two 
sides of the twinning plane correspond to the positive and the 
negative readings. They must be sought on the upper half and 
* A thorough article on this subject, with reproductions of the diagrams of 
Michel Levy has been published by Phofessok N. H. Winchell, this journal, 
vol. xxi, No. 1. 
