1897.] Petrography. 423 
General Notes. 
PETROGRAPHY.' 
Petrography of the Bearpaw Mountains, Montana.—A 
second paper’ by Weed and Pirsson? on the rocks of the Bearpaw 
Mountains describes several intrusive masses in this mountain group, 
a leucite-lava from Bearpaw Peak and a series of the dyke rocks 
occurring so abundantly throughout the region. One of the intrusive 
masses is just north of Wind Butte. It consists of an augite-syenite 
composed of segirite-augite, microperthite and a few accessories, among 
which is sodalite. At the post-office of Lloyd is an intrusive mass of 
trachyte, that has altered the argillites through which it cuts. The 
rock consists of orthoclase phenocrysts in an aggregate of feldspar, 
hornblende, augite in two generations and biotite in two generations. 
The Structure of this groundmass is allotriomorphic, hence the rock is 
as closely allied to the syenite-porphyries as it is to the trachytes. An- 
other intrusion near the trachyte is a nephelite-basalt containing biotite 
and sodalite. The highest point of the mountains, Bearpaw Peak, is 
composed of leucite-basalt lavas, breccias and tuffs. In the midst of 
these rocks is an intercalated flow of leucitite, in which occur pheno- 
crysts of biotite, augite and leucite in a groundmass of the thickly 
crowded leucites in a glass base. An analysis of the rock gives the 
following result : 
SiO, AlO; FeO FeO MgO CaO NaO K,O H,0 gh a POs m MnO BaO SrO Total 
46.51 11.86 7.59 4.39 4.73 7.41 2.39 8.71 3.55 .83 04 22 50 .16—99.73 
The dykes of the region comprise syenite porphyries, leucite basalts, 
tinguaites and minettes. The tinguaites are mainly porphyritic rocks, 
but their phenocrysts are limited to the interiors of the dyke-masses, 
being absent near the peripheries. A tingauite porphyry from near 
Wind Butte is composed of aegirite, augite, alkali-feldspars, nepheline, 
cancrinite and small quantities of apatite, sodalite and fibrous horn- 
blende. The pyroxenes present are made up of cores of an aggregate 
of colorless augites, surrounded by alternate zones of ægerite and augite. 
The large sanidine phenocrysts in the rock are surrounded by mantles 
of xgerine prisms, lying parallel to the banding planes of the feldspars. 
! Edited by Dr. W. S. Bayley, Colby University, Waterville, Me. 
? Cf. AMERICAN NATURALIST, 1896, p. 741. 
* Amer. Jour. Sci., Vol. II, 1896, p. 136 and 188. 
