40 TllC AtllCt'ican (iCoiogisf. Jamiaiy, 18<JS 
by the figures at the intersections of the meridians and the 
parallels. Any line situated in oio may, hence, be taken as 
the axis of a zone, and sections cut in that zone will show the 
extinctions expressed on its principal meridian for the feldspar 
considered. 
These plates are stereographic projections of the sections 
of the several plagioclases perpendicular to the prism. The 
poles of the various planes of this prism are in the circumfer- 
ence. The pole of the base (ooi) is not far from the center, 
in the lower right-hand quadrant. The poles of some other 
planes are also expressed. The entire surface is divided by 
the meridians and parallels into quadrangular areas having 
arcs of five degrees. The meridians running from right to 
left all pass through the poles oio. The parallels which sur- 
round the poles are the various stereographic positions of 
planes inclined in different degrees to oio at intervals of 5", 
but in the zone whose axis is perpendicular to the vertical axis 
c and parallel to 010. These elements are represented by 
the fine black lines as meridians and parallels, and are the 
same for all the plagioclases. The trace of the optic plane is 
shown by the heavy black line passing through the loci of the 
optic axes A and B and the bisectrices % and //p. The other 
heavy lines show the planes connecting :)im with the bisec- 
trices. About the axes of elasticity are double curves in brok- 
en lines. These unite the poles having the same double re- 
fraction. About the optic axes the first curve shows the 
double refraction 0.25. Then comes the curves of 0.35 to 
0.85. This last surrounds the mean axis of elasticity //,„. At 
the principal intersections of the meridians and the parallels 
are figures which give, in degrees, the angles of extinction 
for the planes whose poles are at those intersections. They 
are counted from 0° to 90° and are -\- on the right of the trace 
OIO (the albite line), and — on the left, as indicated. The ob- 
server is supposed to be placed above the epure perpendicular 
to each radius of the hemisphere, his body parallel to the trace 
010; that is to say, to the parallels, his head higher than his 
feet for the lower semicircle, but lower for the upper semi- 
circle. The emergence of the optic axes is at A and B. The 
axes of elasticity //g , ^/„, and n^^ pierce the great sphere at the 
points expressed for each feldspar. They are imited by great 
