62 The American Geohxiist. July, 1895 
cording to views of Tsohermak, in a series from albite to an- 
orthite, called the soda-lime feldspars, in which albite and 
anorthite stand at the extremes, the former representing the 
largest percentage of soda and the latter the largest of lime. 
All the intermediate feldspars are, by this classification, con- 
sidered variable and indeterminable mixtures of albite and 
anorthite. These would include oligoclase, andesine, labra- 
dorite and bytownite, but it excludes microeline which is also 
a triclinic feldspar nearly approaching orthoclase crystalli- 
graphically, and identical with it, according to Michel-Lev}^ 
only differing from it in having evident both albite and 
pericline twinning. 
This theoretical chemical arrangement into a graduated 
series has become very popular as, after its adoption by Ros- 
enbusch, it found its way into most German and English 
works on mineralogy, and is also generally taught in Ameri- 
can schools of petrography. It is probable, however, that, 
while for its convenience it will serve as a useful grouping of 
a lot of facts unknown, it is still destined to be superseded 
in its ordinary interpretation by more exact knowledge. 
There is a certain plausibility also which is apparent in its 
main idea. Nothing is more common than a minute interca- 
lation of two kinds of feldspars. Albite and orthoclase are 
» 
characteristically thus closely intergrown, as in perthite. All 
twinning, even of the plagioclases. seems to be based on 
minute molecular variations which are, as yet, unascertaina- 
ble, but which still probably have their chemical as well as 
their phj'^sical manifestations, and could be presumed to have 
resulted in such chemical variations as the law of Tschermak 
requires. 
Notwithstanding the popular approval of what may be des- 
ignated the German school of petrographers on this subject, 
the nice microscopic researches of Messrs. Fouque, Michel- 
Levy and La Croix, in calling attention to constant physical 
differences between the feldspars of this series have gone di- 
rectly against the idea of their being made up by variable 
mixtures of chemical units, and have demonstrated the valid- 
ity and the constancy of the various species. Microscopic 
research cannot be said to have yet reached the point where 
it traces the chemical atom from place to place and detects its 
