222 The American Geologist. October. i9oo 
Theoretical Composition 
Oxygen . ^ Nomenclature of 
ratio SiOg Al,03 CaO Na^O Sp. Tscher- Schus- Fou- Michel 
1:3 :x " ■ Gr. niak* terf quel Levyg 
Ab4An3 7.2 57.4 27.1 8 
AbjAni 6.7 55.6 28.3 10 
AbsAn^ 6.2 53.7 29.6 II 
AbgAn., 6.0 53.0 30.1 12 
AbjArig 5.6 51. 1 31.2 13 
Ab,An3 5.2 4Q.3 32.6 15 
AbiAn^ 4. 9 48.0 33.4 16 
4 5 
8 4 
3 4 
7 3 
3 2 
3 2 
6 2.684 -■^ndesine Andesine Andesine And'ue 
7 2.694 Labrador Lab'r 
9 2.703 Lab.-Byt. Lb.bas. 
6 2.7o8'-'abrad. Labrad. L,b.Byt. 
8 2.716 
8 2.728 
By town. 
3 2.735 Bytown. Bytown. 
It will be seen at once that the differences of usage are 
very marked. Take for example labrador-bytownite; accord- 
ing to Michel Levy it has the composition Ab2An3; according 
to Fouque it is AbsAm, while Schuster and Tschermak do not 
recognize it as a subspecies, but if they did would probably 
consider it respectively Abi Ana and Abi Ans. Further, Tscher- 
mak and Schuster place the divisions at points whose com- 
position is easily expressed, such as Abi Ani and Abi Ana, 
while Michel Levy, following a method which seems more 
practical, prefers to give names to these points, and place the 
divisions between them. Thus the labradorite of Michel 
Levy is neither distinctly a labradorite, nor an andesine, but 
is at the limit between them according to Tschermak. Fouque 
considers Abi Ana a bytownite, since it is with this feldspar 
that the attack by acids becomes easy; such a marked differ- 
ence should certainly receive recognition. 
For the sake of brevity, the plagioclase of the composition 
expressed by Aba Am will be called labradorite throughout 
this article. It is therefore the same as the labrador-bytownite 
of Fouque, and the basic labradorite of Michel Levy. Ac- 
cording to many published analyses as well as according to the 
optic and crystallographic characters as determined by the 
writer, it is much the commonest feldspar of the plagioclasytes 
*G. Tschermak: Lehrbuch der Mineralogie: Fourth Ed.. Wien. 
1894. p. 473. Theory first proposed: Sitzb. K. K. Akad. Wissensch. 
Wien. Dec. 1864. Vol. L., p. 582. 
tMax Schuster: Ueber die optische Orientirung der Plagioklase: 
Mineral, und Petrogr. Mitt. Tschermak: Second series. III. 1880. p. 
153- 
|F. Fouque: L'Etude des feldspaths des roches volcaniques: Bull. 
Soc. Fr. Min. XVII. 1894. 
§A. Michel Levy: Etude sur la determination des feldspaths: 
Paris: 1894, P- 26-7. Idem. Deuxieme fascicule: Paris, 1896. p. 107. 
