Weed and Pirsson—Igneous Rocks of Montana. 318 
Minette. 
This is shown by a specimen from a dike north of East 
Butte. It is a dull, dark stone-gray color, filled with pheno- 
-erysts of biotite which have a maximum diameter of 5™, 
They are very common, and being arranged in an approximately 
parallel position, they give the rock a facility for cleavage in 
that plane. The lens shows a pale yellowish augite to be also 
present. 
Under the microscope the following minerals are seen: 
Apatite, iron ore, biotite, augite, orthoclase and calcite. 
The biotite is of a very pale leather-brown color and feebly 
pleochroic. It has a zonal structure, being invariably bordered 
by a narrow deep brown rim. The axial angle is extremely 
small and the axial plane is parallel to an edge of the hexagonal 
section ; it is thus a meroxene. The large plates are greatly 
embayed and irregular and frequently composed of smaller 
individuals in parallel position. It thus corresponds precisely 
to the biotite characteristic of minettes as described by Rosen- 
busch.* Apatite is not very common. It presents noth- 
ing unusual. The iron ore is entirely in the second generation. 
The augite is in rather large, stout, ill-formed crystals, colorless 
and with here and there patches of alteration into carbonates. 
The groundmass in which the phenocrysts lie is composed of a 
mixture of very fine biotite leaves and shreds, small grains and 
rods of augite, grains of iron ore and laths of an untwinned 
feldspar which appears to be orthoclase. The orthoclase laths 
are at times arranged in rude spherulitic forms. Often among 
the augite prisms are some that are deep green and strongly 
pleochroic and extended in an optically negative direction. 
They must therefore be of egirite. Between the feldspar 
laths there frequently appears small formless patches of a 
colorless isotropic substance that is believed to be glass. 
The rock has a porphyritic structure and it is a very typical 
minette. It is also quite fresh, there being no alteration except 
in the augite. It contains some fragments of calcite that 
appear to be foreign to it, as if brought up from rocks below 
as an inclusion. In it the dark minerals play a preponderating 
role, the feldspars being present but in comparatively small 
amount in the groundmass. Its petrological affinities ally it 
with the syenite-porphyry of East Butte, with which it probably 
stands in geological relation. From this point of view it is 
interesting to note the occurrence of the egirite both in the 
porphyry and in the minette. 
Washington and New Haven, June, 1895. 
* Mass. Gesteine, p. 310, 1887. 
