242 The Americati Geologist. October, i9uo 
The interference figure in convergent light plainly shows 
the same fact, since blue is found on the concave side of the 
hyperbolas in sections perpendicular to //g ,and on the convex 
side in sections perpendicular to n^ . Further, the examination 
of the interference figures shows that, besides this dispersion 
of the optic axes, there is a weak dispersion of the bisectrices. 
For, when the black cross is formed, blue is found on one side 
of the bar in one optic axis, and on the other side of the bar in 
the other optic axis. This is evidence of crossed dispersion of 
the bisectrices. Further, one optic axis is strictly circular, 
while the other is slightly elliptical; therefore we have slight 
inclined dispersion. Neither kind of dispersion of the bisec- 
trices is strong enough to produce any marked effect on the 
colored rings which are nearly uniform in tint on all sides. 
The position of the optic elements has been the object of 
considerable work by several writers, but owing often to the 
absence of reliable analyses of the material studied, the results 
obtained are discordant, and in general unsatisfactory. Fou- 
que, indeed, has obtained reliable results, but his studies were 
confined to the volcanic rocks, with a single exception. That 
one exception gave him results differing notably from his 
other values. The position of the optic elements is determined 
by measurements of the extinction angles in sections of defi- 
nite orientation. These sections are usually, and preferably, 
those perpendicular to the two bisectrices, and those parallel 
to the two good cleavages, p (ooi), and ^^ (oio). 
Angles of extinction parallel to the easy clcava(je. The 
angles of extinction in the faces p{po\) and^'(oio) are of much 
importance from the point of view of the determination of the 
feldspars. 
In the case of the plagioclasyte from Carlton peak, sections 
and cleavage fragments parallel to ^'■'(oio) give- extinction an- 
gles varying from — 23° to — 2^° , but the commonest angle is 
very near — 26°. Sections and cleavage pieces parallel to 
/(ooi) also show variations, in this case, between — 10° and — 
15° but usually very near — 12°. 
These values are entirely comparable to those obtained by 
Fouque, which are given on the following page. 
Amjles of extinction perpendicular to the bisectrices. Fou- 
que has shown the importance of the value of the angles of 
