292 HAROLD L. ALLING 
This complicates the methods of determining the order of crystal- 
lization of the minerals in a rock. | 
17. Primary perthite, due to the freezing of the eutectic mixture, 
analogous to ledeburite in steels, is probably uncommon in nature. 
18. The potash feldspar of pegmatitic origin, the usual museum 
variety of “‘orthoclase,” is soda microcline and very rarely, if at 
all, ‘‘orthoclase.”’ In fact orthoclase (nearly pure potash feldspar 
without microclinic characteristics) is very rare in nature. 
tg. Some ‘‘adularias”’ and “microclines’’ show microclinic twin- 
ning when examined in thin sections, but in thin plates or in crushed 
fragments they do not exhibit twin striations. The suggestion is 
strong that the pressure, which the specimens experienced in the 
grinding process in the preparation of the thin section, has hastened 
the inversion of the metastable soda orthoclase to soda microcline. 
This raises the question whether complete reliance can be placed 
upon thin sections in the identification of the feldspar species. 
20. That microclinic and orthoclasic feldspars, with a content 
. of the potash component higher than 85 per cent of the whole, are 
exceedingly rare in nature. It is far more accurate to speak of 
soda microcline and soda orthoclase than of microcline and ortho- 
clase. Very frequently specimens of so-called microcline or 
orthoclase are found upon examination to be perthitic as well. 
Before assigning a name to a museum specimen, it should be 
microscopically examined. 
21. That the inversion of soda orthoclase to soda microcline is 
often hastened by the pressure set up by regional or static meta- 
morphism, but that the pressure does not produce soda microcline 
from soda orthoclase; it only initiates and accelerates the change. 
The tendency to change is inherent; the pressure merely starts 
the process. 
22. All plagioclase specimens contain some potash component; 
the average is in the neighborhood of 5 per cent. It is more accu- 
rate to assume that the potash component is present to this extent 
than it is to ignore it altogether. The extinction angles of the 
soda-lime feldspars enable the petrographer to ascertain the amount 
of the soda component present but they do not determine the 
amount of the lime component. The percentage of the potash 
