Detcrminatioi) of tlic Feldspars. — Winch ell. 13 
France, due to the genius of MM. Fouque and Michel-Lev}-, 
whose remarkable contributions have attracted the attention 
of every petrographer.* They not only embody, in the latest 
advanced form, the results of all earlier petrographers, but 
they extend the means of determination to greater scope and 
greater refinement.f If it shall serve to call the attention of 
American petrographers, now under the dominance of the 
German school, to the excellence of the French methods, the 
object of this sketch will be accomplished. 
Preparatory to this it is necessary to recall briefly the prin- 
cipal characters of the feldspars. 
^Following are the titles of the original publications to which refer- 
ence is made above: 
Michel-Levy : — 
De I'emploi du microscope polarisant a lumiere parallel pour I'etude 
des plaques minces des roches eruptives. Ann. d. Mines, Dec. 1877. 
Mesure du pouvoir birefringent et positions d'egale intensite lunii- 
neuse des mineraux en plaque mince, 1884. Bui. See. Min. France. 
Mineraux des roches (with A. Lacroix), 1888, Baudry & Cie., Paris. 
Sur les moyens: (i) de reconnaitre les sections paralleles a g' (010) 
des feldspaths dans les plaques minces; (2) d'en utiliser les proprietes 
optiques. Comptes Rendus des Seances de 1' Academic des Sciences, 
t. CXI., p. 700, 1890. 
Etudes sur les roches des Puys et du Mont-Dore. Bul. Soc. Geol. 
France, Reunion extraordinaire a Clermont-Ferrand, 1890, p. 674 et 
suiv. Plusieurs Contributions. 
Etudes sur la determination des feldspaths dans lea plaques minces 
au point de vue de la classification des roches. Baudry & Cie., Paris, 
1894, p. 171. 8 planches. 
Diifo .•— 
Deuxieme fascicule. Sur i'eclairement commun des plagioclases 
zones: Proprietes optiques de microcline. Baudry & Cie., Paris, 1896. 
Fouque :- - 
Mineralogie micrographique. Roches eruptives frangaises (avec 
Michel-Levy). Mem. de la carte geol. de France. Ministerc des Tra- 
vaux publiques. Paris, 1879. 
Contribution a I'etude des feldspaths des roches volcaniques. Bul. 
Soc. Min. Fran., 1894. 
Lacroix : — 
Lastly, these principles and methods have been extensively applied 
by Prof. A. Lacroix in his late works: Les enclaves des roches vol- 
caniques (Ann. Acad. Macon, vol. X, 1893), and Mineralogie de la 
France et de ses colonies (Paris: Baudry & Co., 1893-6); where will also 
be found a large number of new optic properties of the various minerals, 
many of them pertaining to the feldspars. 
tThe writer is under great obligation to Messrs. Michel-Levy, 
Fouque and Lacroix for assistance and critical suggestions in the prepa- 
ration of this sketch, and to Dr. U. S. Grant in its revision. 
