1894.] Mineralogy. 875 
the two minerals with a cone of illumination of small angle. When 
properly focused, this contact appears asa sharp line. On raising the 
tube of the instrument, the focus is disturbed and a light band appears 
on the side of the contact toward the more refractive mineral, which 
band widens and finally fades out as the tube is raised higher. If, on 
the other hand, the tube be lowered, the same phenomena appear on 
the other side of the contact. The best results are obtained with the 
use of high powers and with a cone of illumination of small angle. 
Becke recommends the use of the 7risblende furnished with the newer 
instruments of Fuess. I have obtained good results with a small 
Voigt and Hochgesang instrument by removing the weak convex 
lens which covers the polarizer. Becke’s Schlierenmethode makes use 
of inclined illumination, which is obtained with the Jrisblende or 
with Abbe's Beleuchtungsapparat. With inclined illumination, that 
side of a section of strongly refracting mineral toward the direetion 
from which thelight comes, shows a light band against the less strongly 
refracting mineral surrounding it, while the opposite side shows a dark 
band. The author statesthat this method suffices to determine ortho- 
clase, quartz, and a plagioclase when they are present together in a 
holocrystalline rock, but suggests that it be supplemented by the 
Fürbung method. The method of determining the species of plagio- 
clase depends on the comparison of the double refraction of the feld- 
spar with that of quartz sections. By making per cents of An the 
abscissæ, and indices of refraction the ordinates, curves are obtained 
for a, f£ and y within the feldspar series. These curves are inter- 
sected by the horizontal curves of w and e in quartz. If now a and 
y be the less and the greater values respectively of the refraction for the 
two principal directions in any section of plagioclase, a' being between a 
and 5 and y between £ and y, the curves obtained indicate the follow 
ing relations: 
Parallel Position Crossed Position Composition. 
ec cv boy oy c7» z2' Ab  —Ab, An, 
ll: w> 6! eom y E d € >’ Ab, An, —Ab,An, 
Hb ose ey o xy e>. Ab, An, —Ab,An, 
IV wae’ e=7 omy ee Ab, An—Ab, An, 
Vaan’ y o< yf e==c’ Ab,An,—Ab An, 
Vl oxu’ e« y o< y e<e’ Ab, An—An, 
It is seen that these subdivisions of the plagioclases correspond in a 
general way to the earlier one of Tschermak, I being albite, II and III 
oligoclase, IV and V andesine, while VI includes labradorite, bytoun- 
ite and anorthite. As Tschermak's later and more equable subdivi- 
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