Gabbroid Rocks of Minnesota. — Wincliell. 299 
dierite in color, lustre and transparency is entirely comparable 
to clear quartz. The refringence is low, but in this rock: 
Cordierite n^ > n^ quartz. 
Cordierite n^ < ?i„^ labradorite. 
The birefringence is slightly higher than that of quartz 
and gives pale yelloAv as the maximum interference color. The 
mineral is negative, and about ?ip the optic angle is too large 
to permit measurement with an ordinary microscope. The 
elongation is negative with extinction parallel. 
Practically the only way to make a certain determination 
is to search with high powers, (since the grains and crystals 
are very small in this rock), to find the twinning characteristic 
of the mineral. 
There are two methods of twinning: in both the vertical axis 
is the twinning axis, about which the rotation is 60°. lathe 
first the face of association is ;/?(iio) and in the second it is 
^'(130). Both types produce pseudohexagonal forms when 
repeated. They may also give rise by interpenetration to 
polysynthetic bands very similar to those of the feldspars. 
Such bands may be seen in sections parallel to /(ooi), and also 
in sections parallel to ^'(010) and /i\ioo). In the first case 
they show symmetrical extinction of 30° ; in the second case 
they extinguish together parallel witli the twinning line, but 
show different birefringence. 
A section parallel to /'(ooi) showing either type of twin- 
ning is sufBcient to distinguish cordierite from all other 
minerals, including the feldspars. For, in the cordierite, every 
individual of the twin will be. strictly perpendicular to np, and 
the extinction will be symmetrical, at 30° from the twinning 
line, while among feldspars the only ones which can show ?/p 
sensibly perpendicular in every individual are oligoclase and 
oligoclase-andesinc. in which the symmetrical extinction on 
each side of the twinning line cannot exceed 7° or 8°. 
The optical orientation in the first type of twinning is 
shown in Plate XVIII, Fig. 17, while Fig. 18 shows the seconrl 
type, as well as twinning by interpenetration; both figures 
were drawn with a camera lucida from thin sections of the 
cordierite noryte (983). 
When crystal outline is present, sections perpendicular to 
//if, the obtuse bisectrix, can be distinguished from other color- 
