MUSEUM OF COMPARATIVE ZOOLOGY. 229 
the presence of Cl, Na, SO,, and a little Ca; so that there is a mixture 
of the hauynite molecule. ‘The specific gravity of an isolated piece was 
2.19, which is low for sodalite (2.28), and must be attributed to zeoliti- 
zation. W. Lindgren has described,’ under the name “ analcite-basalts,” 
basic rocks from the Highwoods (a range lying north of the Crazy 
Mountains and a locus of similar magmas), composed of olivine, mag- 
netite, a little biotite, and a mineral determined as analcite, but no feld- 
spar or.nepheline. The analcite appears in the slides in hexagonal or 
octagonal sections, clear and isotropic, and was determined to be such 
by specific gravity and chemical analysis. Lindgren considers this pri- 
mary. Judging from the descriptions, there is some resemblance to the 
mineral identified by us in the present rock, without quantitative anal- 
ysis, as sodalite ; but as the latter has the form and other properties of 
sodalite, and is identical with that mineral as occurring in the granular 
and porphyritic theralite, this seems the correct determination, 
Augite.— The porphyritic augite has planes developed in the pris- 
matic zone, sometimes terminal, and the common orthopinacoidal twin- 
ning. In sections the pale green of the centre of the crystals gradually 
increases in depth towards the edge, which is formed of deep green 
eegirine, as in the theralites, having the characteristic small obliquity of 
extinction in the prismatic zone and axis of elasticity, a, near the vertical 
axis. The augite is generally fresh, and includes crystals of magnetite 
and sodalite. 
The slides contain occasional plates of biotite and grains of magnetite. 
Groundmass of the Acmite-trachyte Type.— This is composed essen- 
tially of slender lathe-shaped feldspars and acicular crystals of egirine, 
which lie either between the feldspars or included in them, and by 
preference in the outer portion. These minerals appear in sections 
transverse to the flow with perfect fluidal arrangement ; but when cut 
parallel, many of the feldspar sections are broad and rounded, and contain 
the exgirine needles in a network without parallel arrangement. The 
feldspars are therefore somewhat tabular in the plane of flow. Single 
crystals, Carlsbad twinns, or multiply twinned crystals occur, with ex- 
tinctions parallel or but slightly oblique to their length, and are doubt- 
less also anorthoclase in part. With the green exgirine a few brown 
-needles of acmite occur. Apatite and magnetite are found in the usual 
form. The combination of nearly parallel feldspar and zgirine needles 
gives the former a peculiar feathery look in polarized light with low 
powers. 
1 “ Eruptive Rocks from Montana,” Proe. Cal. Acad. Sci., Vol. III. p. 51. 
