36 Tlic American Geologist. January, i898 
be brought to the position of tangency at tlie margin of the 
field of the microscope, first on one bisectrix and then on the 
other. In each case the amount of rotation is noted in degrees 
at the margin of the platine. That which requires the greater 
amount of rotation to produce tangency is the axis of the 
acute angle. Finally, when there remains uncertainty whether 
the angle is acute or obtuse, it may be measured by the use 
of the axial goniometer already described in the American 
Geologist.* 
Once the interference figure is well centered and the angle 
known, it remains to measure the extinction angle with a trace 
of a known crystallographic character. This angle is that 
made by the axial plane with a twinning (albite) lamella, or 
with the cleavage parallel to ooi. Sections perpendicular 
to Wp are always measured, for this angle, on the albite twin- 
ning, or, which is the same thing, on the cleavage oio. In 
the case of sections perpendicular to % the same crystallo- 
graphic character is employed in examining the basic feld- 
spars, but it is necessary to have recourse to some other char- 
acter for the acid feldspars in which the plane perpendicular to 
//g is parallel, or nearly parallel, to the face oio, rendering the 
cleavage (oio) and the albite made invisible. In that case 
extinction is measured on the cleavage parallel to ooi, which 
is very rarely wanting in the acid feldspars. In all the lime- 
soda feldspars, except albite and anorthite, the sections parallel 
to ooi and oio, and perpendicular to ng, , appertain practically 
to the same zone. For these feldspars the extinction angle 
upon a section perpendicular to //jr has therefore the same 
value, whatever be the cleavage, that of ooi or that of oio, 
which is chosen for the measurement of extinction. 
The measurement having been taken, reference may be 
made to the following table, given by M. Fouque as a sum- 
mary statement of the results of all his work. The beginner 
may be warned against the liability of misreading the extinc- 
tion angle, thus getting the complement of the extinction angle 
given in this table. In that case his error consists in measur- 
ing, not from the trace of the axial plane, but from a line per- 
pendicular to it. In other words — the axial plane, i. e., the 
position of extinction, should be made to coincide with the 
*Op. cit., Vol. XVII, p. 79- 
