Determination of the Feldspars. — WincJicll. 3:; 
more feldspars in the same thin section, and (2) the possible 
non-existence of the maximum equal extinction in any of the 
crystals cut by the random section. The former is more likely 
to arise in the examination of the acid and metamorphic rocks, 
and the latter in case of a limited number of feldspar sections 
in the rock cut. In the presence of two or more feldspars, 
however, usually they will be found to dififer in transparency 
or in mode of distribution, or in other evident optical charac- 
ters, and the error can be obviated. In case of the feldspathic 
microlites, they are almost invariably of the same species when 
formed rapidly at the second consolidation. The second obsta- 
cle can only be reduced by increasing the number of feldspar 
sections subjected to inspection. 
These maxima, alone, are sufficient to identify the oligo- 
clases (0° to 5°), the basic andesines (more than 16°, less than 
22°), the labradorites (from 22° to 35°), the bytownites (from 
35° to 45°), and the anorthites (above 45°). 
When the feldspar microlites are twinned on the albite 
plan, as frequently happens, they are amenable to this process 
of examination. When they are simple their determination 
is more difficult. It has been proposed by Michel-Levy, 'in 
that case, to employ the zone 001 :oio parallel to the axis of 
which they have their longest dimension, but the results ob- 
tained are not sufficiently characteristic for all the species. 
Extinctions of such microlites, cut in this zone and referred to 
their longer dimension, are as follows: 
Orthoclase 0° to 5° 
Microcline 0° to 16° 
Albite 0° to 20° 
Oligoclase Ab4 Ani 0° to ^° 
Oligoclase Abs Ani 0° to 0° }■ Compare p. 43. 
Andesine 0° to 7° 
Labradorite 0° to 18' 
Labradorite (basic) .... 0° to 32' 
Anorthite 0° to 55' 
Microcline, some forms of labradorite and albite could 
hardly be distinguished by their maximum extinctions in 
simple microlites, but orthoclase, oligoclase and anorthite are 
characterized by maxima which are sufficiently distinct. 
Microcline, however, is rarely or never seen in the condition 
of microlites, while the associations of labradorite and albite 
