Weed and Pirsson—Igneous Rocks of Montana. 311 
Quartz Diorite Porphyrite. 
This rock is represented by two specimens from West Butte. 
It is dense, of a dark gray color and thickly spotted with white 
feldspar phenocrysts. They are generally equidimensional 
and about 2™™ across, but in one specimen examples of twice 
this size occur. At times they are somewhat elongated and 
are then arranged in flowage planes showing fluid movements. 
Occasional small crystals of hornblende up to 4™™ in length can 
be seen. The rock weathers with a brownish crust. 
Microscopically the following minerals are observed to be 
present: Apatite, hornblende, iron ore, plagioclase, orthoclase 
and quartz. 
Plagioclase. As indicated in the hand specimens, this is 
very abundant as a phenocryst. The crystals are generally 
well formed and tabular on 0(010). They are twinned accord- 
ing to the albite and Carlsbad laws. They are usually zonally 
built and the more basic inner portion shows a perceptible dis- 
persion of the optic axes. They vary in composition from a 
basic labradorite to a mediuin acid oligoclase; thus a section 
in the zone a(100) on c(001) gave extinctions on either side of 
the albite twinning line of 26°, while the Carlsbad twin gave 
12°, thus showing a basic labradorite approaching dytownite—a 
determination confirmed on other sections. In these sections 
the zonal growth is greatly marked and steadily becomes more 
acid towards the periphery. In another specimen the feld- 
spars are more acid and even approach acid oligoclase. 
The hornblende is pretty common in short, stont, ill-formed 
erystals and is nearly always changed to masses of opacite and 
chlorite and other decomposition products. It is the usual 
dark green variety. Twins at times on a{100). 
The groundmass in which the above minerals lie is a very 
fine-grained, patchy or micro-poikilitie mixture of quartz and 
feldspar, the latter usually too much altered or kaolinized for 
identification but certainly composed in part of orthoclase. 
The quartz occurs also at times in irregular grains, which rise 
to the position of small phenocrysts. A greenish-yellowish 
mineral of the chlorite group occurs very often in cavities 
arranged in vermicular growths. 
Quartz Syenite Porphyry. 
This rock is represented by two specimens from East Butte. 
It is tongh and compact, of a pale brown color which changes 
to a pale green gray on weathered surfaces. This groundmass 
is very thickly crowded with phenocrysts of feldspar which 
vary very greatly in size from examples 11™™ by 6™™ down to 
some only 0°5™™ across. They have the common flesh color of 
