34 Tlie American Geologist. January, i898 
are so dififerent that there is httle danger of confounding them. 
Labradorite is the commonest product of the consoHdation, 
of the basic eruptives and albite almost invariably results from 
metamorphism, frequently from the contact of igneous rocks 
on the calcareous elastics. 
There is very little reason to expect, side by side, micro- 
lites of dififerent natures. In the vast majority of cases the 
microlites are formed rapidly, and present a great preponder- 
ance of a single species of plagioclase. 
(<:•) Sections Perpendicular to the Bisectrices. 
This method of determination requires the use of con- 
vergent light and the careful observation of the interference 
figure of the axis of elasticity. It is well, also, but not always 
necessary, to place a drop of iodide of methyl, or of glycerine, 
on the lens of the objective, and another on the upper lens of 
the condenser, in order that when they are both brought near 
the slide holding the section, the liquid will spread to the right 
and left, producing practical immersion. This increases the 
field of possible observation without deranging the geometric 
relations. 
J. he details of this method have recently been elaborated 
by M. Fouque,* who has confirmed it by a great number of 
illustrations, and by chemical analyses. It may at first sight 
appear to be a difificult task to obtain sections perpendicular 
to the bisectrices % or Wp. But when it is remembered that 
cleavage pieces or sections cut parallel to oio will nearly 
always show the axis ng , and when not perpendicular may be 
made so by a little oblique grinding or by the use of the tilting 
stage of Federov constructed by Nachet, and also that the 
axis n^ is in the vicinity of the edge ooi : oio, it is evident 
that but little manipulation is necessary to cut a crystal per- 
pendicular to either axis. Small transparent crystals are neces- 
sary, or pieces of larger crystals bounded by known cleavages. 
The little crystal is encased in a ball of thick Canada balsam 
which can be moulded at will. This is allowed to swim in a 
Balsam more liquid, enveloped in a little glass ring. Observed 
thus in convergent polarized light the crystal is brought to 
^Contribution a I'etude des feldspaths des roches volcaniques. Bui. 
Soc. Min. France, Vol. XVII, 1894, pp. 283-611. Also issued sepa- 
rately with independent paging. 
