12 The A})ieiican Gcolonst. January, l«l^ 
THE DETERMINATION OF THE FELDSPARS. 
By N. H. WiNCHELL, Minneapolis. 
C'est par des efforts de ce geure que nos m6thodes micrographiques finiront par 
conqu6rir la place qui leur est due dans les programmes universitaires et dans I'eu- 
seignement desgrandes kcdies.—Fouque. 
C'est un devoir pour tout p6trographe qui se livre k une 6tude uouvelle do s'inge- 
nier pour amender et porfectionner les proc§d6s de reclierche que lui douue la sci- 
ence coutemporaine. — Fouque. 
La d6termination precise, rapide et relativement facile de tous les 616ments 
foldspatliiques des rocliesestune des conditions indispensable h, I'^tablissement d'une 
classification rationelle.— il/;"c/ieM,^t7/. 
On peut en quelque sorte jauger la valeur d'une 6tude petrographique au soiii 
apport6 par I'auteur k determiner les feidspatlis do ses plaques minces. — Michel- 
Levy. 
Introduction. 
Owing to the prevalence of the feldspars in nearly all crys- 
talline rocks their accurate determination is one of the essen- 
tials in practical petrography. As they appertain to the mono- 
clinic and the triclinic systems, their investigation involves 
most of the problems which arise in the use of the petrograph- 
ical microscope. Hence, as has been remarked by Michel- 
Levy* the progress of accurate methods of their determina- 
tion has been the touchstone of progress in microscopical 
petrography. This progress is due to the skill of several petro- 
graphers — Descloiseaux, Fouque. Michel-Levy, Schuster, 
Federov and others. 
The following sketch consists of a presentation chiefly of 
the methods which have been devised within recent years in 
^Determination des feldspaths dans les plaques minces, 1894, p. 2. 
