1 78 The Amenca?i Geologist. September, 1900 
deed even with the immersion objective (Nachet No. 7) the 
position of the optic axes was determined only after long and 
careful search. It is, however, readily determined that the 
elongation of the fibers is always positive and that the re- 
fringence is always notably higher than that of the labradorite. 
The fibers rarely, if ever, attain the thickness of the section, 
and are usually less than half that thickness; nevertheless, it is 
safe to assert thaf the birefringence is at least .030. The 
zeolitic extinction of the mineral prevents an accurate deter- 
mination of the extinction angle of the fibers; but it is clearly 
never higher than three or four degrees, and may be always 
zero. 
The only rock-forming minerals known to the writer which 
possess all the characters enumerated above are pyrophyllite, 
talc, certain micas and pectolite. In all these minerals fi^ 
( = a ) is nearly perpendicular to the easy cleavage/'(GQi), but 
the optic angle is very distinctive for each one, thus 2E in talc 
is 15° to 20° ; in muscovite it is rarely less than 50° and never 
more than 75°; in pyrophyllite it is 105° to 110°, while in pecto- 
lite it is 143°, being in the last case the value of the apparent 
obtuse optic angle, since pectolite is positive. A section was 
finally found very nearly parallel to the easy cleavage, and at 
the same time large enough to give a good interference figure. 
The figure obtained was a distinct bisectrix distant about 
two-thirds of the radius from the center of the field, but show- 
ing that the section was strictly perpendicular to the optic 
plane. The quartz and mica plates both showed the section lu 
be perpendicular to 7iv (=a) which must be the obtuse bisec- 
trix since the hyperbolas leave the field very promptly. In- 
deed the angle between the tangent positions of the hyperbola 
on the two sides of the field is only about 2)2>° i^ spite of the 
fact that the obliquity of the section must increase this angle. 
Therefore the mineral is positive and must be pectolite. 
It is unfortunate that the mineral does not occur in suf- 
ficient quantity to make a chemical analysis possible ; but even 
microchemical- tests are utterly out of the question. The 
chemical changes necessary for such a transformation are 
readily seen to be the elimination of the alumina with a notable 
decrease in the silica. 
The beginning of the alteration of the pyroxene is often 
