44 TJie Aiiicj'ican Geologist. January, i898 
{g) Means of Discovering Sectio7is Parallel to oio* 
(Compare p. 29, ct suiv.) 
The remarkable work of Max Schuster upon the plagio- 
classes,f so happily completed by the theoretical application 
by Mallard,^ has brought to light what can be drawn from 
the extinctions on any face whatever, provided its direction 
can be determined. 
From this point of view the face 010 is the most con- 
venient, because it eliminates the albite twinning characters, 
and,' further, it distributes the extinctions between numbers 
sufficiently separated. The adjoining diagram (fig. 26) repro- 
duces the curve of Max Schuster, and from that as a datum 
adds the feldspars from the new epures (plates I-VII). 
In order to utilize§ extinctions in thin sections when the 
feldspars appear scattered in a section cut at random, it is 
necessary (i) to know how to recognize sections near 010, 
and to judge approximately of the error committed by de- 
fective orientation; (2) to be able to determine the obtuse 
angle 001 A lOO. 
I. a. The sections 010 being parallel to the face of the 
association of the made of albite, the hemitropic lamellae 
and their overlapping edges ought to widen out, and even to 
disappear at last, at the same time that their extinctions be- 
come the same; for the ellipse of the indices returns upon itself 
after a rotation of 180°. If in a section of the thickness of 
0.02mm (e) the overlapping o£ the two lamellae reaches, for 
example, a width (1) of 0.3mm, we should have evidently, call- 
ing a the angle which 010 makes with the thin section. 
tan a equals-^ equals -Qj^.V- equals yV 
a is less than 4°. We shall see later the error that 
would be committed, from this, in the reading of the extinc- 
tions. 
b. The Carlsbad made has also for face of association 
*Translated from Michel-Levy. Deter, des Feldspaths, p. 46. 
t Uber die optische Orientirung der Plagfoclase, Tschermak. M. 
P. Mittheil, 1880, III, 117. 
X Sur risomorphisme des feldspaths tricliniques. But. Soc. Min. 
de France, 1881, IV, 96. 
§ Comptes Rendus, 10 Nov., 1890. Bui. Soc. Geol. France, 1890. 
Reunion a Clermont-Ferrand, Etudes sur les roches des Puys et du 
Mont Dore. 
