3D2 TJie American Geologist. Xovember, i90o 
is easily distinguished from the pyroxene by its stronger bire- 
fringence which gives color as high as blue: 
**« — *'m ^ -013 at least. 
*<g — «p -= .023 at least. 
The refringence is always lower than that of bronzite, but 
higher than that of biotite. Anthophyllite is biaxial and posi- 
tive with a large optic angle. The direction of the fibers is 
positive. The color is scarcely noticeable in thin section, with 
very feeble pleochroism as follows: 
«g =' pale green. 
«m "=■ pale yellowish green. 
;/p = pale yellowish green. 
Absorption n^ > «ni = ^'p. 
Anthophyllite shows rather feeble pleochroic halos about 
inclusions of zircon. These intensify the ordinary color of 
anthophyllite. 
Muscovite is very rare. It occurs in microscopic lamellae 
as an alteration product. 
One more decomposition product occurs along fractures 
in the rock. The color is clear yellow to orange reddish, 
without perceptible pleochroism. The refringence is distinctly 
higher than that of plagioclase and lower than that of biotite. 
The birefringence is at least .010 and may be considerably 
more as the mineral is rarely as thick as the section. It is 
positive and uniaxial, or more probably biaxial, with a very 
small optic angle. The elongation of the fibers is positive 
with extinction nearly parallel. It is not sensibly attacked 
by either HCl or HiSOs and consequently some attempted 
micro^chemical tests failed, leaving the mineral undetermined. 
The CHEMICAL cOiMPosiTiON of the cordierite noryte shows 
much iron and alumina, and little calcium. For comparison 
two analyses are added of the cordierite norytes described by 
Prof. Lacroix. It will be noted that the Minnesota rock is 
distinctly more basic than those of Pallet. A further dis- 
cussion of this analysis will be found in chapter X. 
The fourth analysis represents a Minnesota rock, which 
can be considered to represent a stage intermediate between 
the normal gabbro, and the noryte studied in this article. It 
is less acid and less highly aluminous; on the other hand, it 
