THE MINERALOGRAPHY OF THE FELDSPARS 275 
QUANTITATIVE MICROSCOPIC ANALYSIS 
Phase Percentage| K-feldspar | Na-feldspar | Ca-feldspar Silica 
morasimalbDlten.csck des aoe. bos 80 8 90 2 fo) 
Primary soda microcline.... . 10 87 se) B ° 
Primary oligoclase.......... 2 17 80 3B ° 
Ml HIME IEC edie oie. Sa aleve 2 09) 75 3 5 
MOMMA Zeist 2 ese tie cieeeve races 3 ° ° ° I0o 
Secondary soda microcline.... 2 89 8 3 ° 
ovashvallottes. 22.2 nce. eco slee ens oes 6.4 71.0 1.6 fo) 
Primary soda microcline.....]........ 8.7 0 8} fo) 
Primary oligoclase.,.......:-||.4-.- i 34 1.6 -06 ° 
MMpvnmieksibe qe acc .s ete dee niieeaece «© 34 it§ - 06 sit 
BP Te gts Zep e ea ar Sut are sa eA cou il iach wa abe soe) lay anes cheer Shandy | axeest ar ovenay eyes 3-0 
Secondary soda microcline...|....... 2 D5] .20 09 ° 
OGEILS ois Gams ect Citroen aecad eae 265] 76.30 2 Bot 
Classification: 
Popular: Antiperthite. 
Technical: Hyperperthite. 
APPLICATIONS OF THE MINERALOGRAPHY OF THE FELDSPARS 
TO GEOLOGICAL PROBLEMS 
CASE ONE—LOCATION OF A FAULT 
The Problem.—One of the graphite properties in the Adirondack 
Mountains visited by the writer in 1917! had been abandoned 
‘because the ore was cut off by a fault. The writer had this infor- 
mation when he entered the field, but to his unpleasant surprise 
he was unable to locate with any satisfaction the faults even 
though slickensided surfaces were found on the walls of the old 
workings. 
As it was deemed very desirable to locate the faults with some 
degree of accuracy, the writer carried the problem into the labora- 
tory for solution. 
Method of Attack.—Preliminary examination of the graphite- 
bearing schist showed that it was chiefly composed of potassic 
feldspar. The suggestion of Rosenbusch? that the development of 
microcline structure in orthoclase is due to pressure was recalled. 
tH. L. Alling, New York State Museum Bull. 199, pp. 61, 68-70, 1918. 
2 Rosenbusch-Iddings, Microscopic Physiography of the Rock Making Minerals, 
Dr 320. 
