258 HAROLD L. ALLING 
MICROSCOPIC EXAMINATION OF NATURAL FELDSPARS 
The present section contains the results of microscopic and 
chemical analyses of typical feldspars from many parts of the world. 
The optical properties of the feldspars were only determined to 
the extent necessary for an identification of the species. The 
extinction angles on (oro) and (oor) were determined by Michel- 
Levy’s “statistical”? method* on crushed fragments, which were 
properly sized by passing through sieves of 100 mesh, caught on 
screens of 120 mesh, and then placed in a suitable mounting fluid 
—such as clove oil. A large share of the determinations were 
made with monochromatic (sodium) light and the stage of the 
petrographic microscope rotated from each position to extinction. 
The recorded results are the averages of five readings. The 
orientation of the fragments was ascertained in the following 
manner. The faces were recognized by the cleavage, the inter- 
ference figure, and the twinning. The fragments broken into 
plates parallel to the base (cor) were found to be more common 
than those parallel to (o10); the former were often identified by 
traces of the albite twinning. The (oro) faces are more likely to 
have parallel edges due to the cleavage. | 
For this experimental work the writer naturally chose speci- 
mens of feldspars whose chemical composition was known, and 
whose chemical analyses were later recast into percentages of the 
three feldspar components. Upon this basis the interrelationship 
of the optical properties and the composition were ascertained 
which enabled the writer to draw the diagrams of the extinction 
angles of the system. The reader must remember that the close 
check between the chemical analyses and that inferred from the 
optical characters is the result of using the chemical analyses for 
the purpose of establishing the relations between chemical com- 
position and physical properties, and so lead to methods of iden- 
tification. 
The percentage of the feldspar phases, when more than one was 
present in the specimens, was determined by a method analogous 
* Michel-Levy, ‘“‘De Vemploi du microscope polarisant a lumiére parallele pour 
Vétude des plaques minces les roches eruptives,” Ann. des Mines (December, 1877), 
PP: 392; 471. 
