234 HAROLD L. ALLING 
He furthermore puts in parentheses after “anorthoclase,” ‘soda 
microcline’’* and ‘‘microcline albite.”? A similar procedure is 
followed by Iddings* who speaks of the potash-soda feldspars rich 
in soda as ‘‘soda microcline (anorthoclase).”’ 
It seems to the writer that this confusion is due to the failure 
to recognize the possible dimorphism of each of the components in 
the series, that is, there may be two distinct binary systems in one: 
the so-called monoclinic series with orthoclase and barbierite as 
end members, and the triclinic potash-soda feldspars with micro- 
cline and albite as components. If this is recognized then it is 
logical that the two should be classified as follows: 
Monoclinic: Orthoclase, soda orthoclase, monoclinic anortho- 
clase, potash barbierite, and barbierite. 
Triclinic: Microcline, soda microcline, triclinic anorthoclase, 
potash albite, and albite. The interrelationship that exists 
between the monoclinic and triclinic anorthoclases has been pointed 
out by Dana‘ who says that the axial angle of anorthoclase varies 
with the temperature, ‘“becoming monoclinic in optical symmetry 
between 86° and 264°C. but again triclinic on cooling. This is 
true of those containing a little calcium” (anorthite). Although 
the writer is somewhat cautious in proposing new terms—for the 
literature of mineralogy and petrography is already burdened with 
many useless names, some of which are worse than useless—yet 
there has been no systematic attempt to subdivide the potash-soda 
series into definite ranges analogous to oligoclase, andesine, labra- 
dorite, etc. Calkins’ decimal principle as applied to the plagio- 
clase series is so logical that its application to the potash-soda series 
is worth attempting. 
The terms hypoperthite and hyperperthite may appear strange 
to the petrographer but the metallographer will recognize old 
friends. ‘The nomenclature proposed is an adaptation of the terms 
hypoeutectoid and hypereutectoid as applied to steels. The word 
eutectoperthite is self-explanatory. If such refinement in classify- 
ing the system is neither possible nor desirable, then perthite can 
t Natronmikroklin. 2 Mikroklinalbite. 
3 Joseph P. Iddings, Rock Minerals, 1911, p. 235. 
4 James D. Dana, System of Mineralogy, sixth ed., 1892, p. 324. 
