1896.] Petrography. 741 
tetragonal class (hemimorphic hemihedral division of the tetragonal 
system). So far as known, circular polarization is not exhibited by 
crystals with this grade of symmetry. 
PETROGRAPHY:! . 
Petrography of the Bearpaw Mountains, Montana.—The 
Bearpaw Mountains are the dissected remains of a group of Tertiary 
volcanoes. Their cores of the old volcanoes are granular rocks, their 
lavas and tuffs are represented by basic sheets and beds. The lavas 
are largely basalts, leucite-basalt and other similar basic types.” 
The cores consist of mica-trachytes, quartz-syenite, porphyries, con- 
taining aegerite-augite and anothoclase-phenocrysts, in which are im- 
bedded microlites of oligoclase, trachytes containing hornblende and 
diopside and shonkinite. A few miles from Bearpaw Peak a denuded 
eore is exposed, which furnishes a good example of the differentiation 
of a syenitein place. The intrusion is laccolitic in character. Around 
its borders it has highly altered the sedimentary rocks with which it is 
in contact. The most acid portion of the laccolite is a light aplitic 
syenite containing quartz and diopside. The main mass is a more basic 
syenite resembling monzonite or yogoite. It contains diopside and 
much plagioclase. The most basic phase isa shonkinite. Analyses 
for the three principal types follow: 
SiO, Al,O, Fe,O, FeO MgO CaO Na,O K,O H,O, Other Total 
Quartz-syenite 68.34 15.32 1.90 .84 .54 .92 545 5.62 45 57 = 99.95 
Monzonite 52.81 15.66 3.06 4.76 4.99 7.57 3.60 4.84 1.09 1.86 =100.24 
Shonkinite 50.00 9.87 3.46 5.0111.92 8.31 2.41 5.02 1.33 2.68 —100.01 
The totals corrected for Fe and Ce are 99.94, 100.22 and 99,93 respec- 
tively, 
Two French Rocks.—In the serpentine of St. Préjet-Armadon, 
Haute-Loire, France, Lacrou® finds nodules composed of asbestiform 
gedrite surrounding a kernel of serpentine or biotite. The nodules are 
separated from the serpentine by an envelope of biotite. They are sup- 
! Edi Colby University, Waterville, Me. 
3 rd bea cng T IV, Vol. 1, p. 283 and 351. 
3 Bull. Soc, Franc. d. Min., XIX, p. 687. 
