2,76 The American Geologist. '^""^' ^'^'^^• 
THE DETERMINATION OF THE FELDSPARS 
IN THIN SECTION. 
By J. E. Spurk, Washington, D. C. 
The accurate determination of the feldspars is of the great- 
est importance in accurate petrography. We have, unfortun- 
ately, no single publication where the different methods of op- 
tical investigation of minerals are given briefly and clearly for 
the guidance of workers without being buried in a mass of in- 
teresting but superfluous explanations. For this reason the 
writer long ago drew up certain 'summaries for his own use 
in microscopic work and he has found these of such value that 
he has been tempted to publish them for the help of others 
similarly situated. The short descriptions were moistly drawn 
up while the writer was studying the subject of feldspar de- 
termination in Germany and France. There is, of course, ab- 
solutely nothing original in them. 
Becke Method of Determining Minerals, Especially Feldspar, 
by Their Single Refraction. 
In any rock section find among the grains of the mineral 
to be studied, one with known orientation, — e.g. feldspar cut 
perpendicular to a bisectrix, as determined by convergent light. 
This grain must be in contact wath a grain of some other min- 
eral, preferably quartz, which should also have preferably a 
known orientation, although in quartz this is not so weighty 
as in biaxial minerals, since in every section the refraction par- 
allel to the axis of least elasticity (w) is constant. "pna^^ 
1.544.* Focus a high power lens on the contact of the two 
minerals. With raising of the lens, a dark band of total re- 
flection migrates in the direction of the minerals of the highest 
refraction. This permits comparison of elasticities. Let pol- 
arized light from the lower nicol vibrate in the direction of an 
axis of elasticity in each mineral, of which axes the relative 
value (to other directions in same mineral) is known; observe 
which index of refraction is the greater ; turn the section 90° 
* Hp (n petit) is the French sign for the direction of least elasticity; n„ (a 
grand) is the direction of greatest elasticitj'; 73^^ (n moycn) is the direction ol 
mean elasticity; «„ signifies the refraction parallel to the direction of least 
Pna 
elasticity, as obserTed in ordinary (sodium) light 
