312 Weed and Pirsson—Lgneous Locks of Montana. 
orthoclase. There are also many little crystals of black augite 
smaller than the smallest feldspar phenocrysts. 
The microscope shows the following minerals present: 
Zircon, iron ore, apatite, eegerine-augite, oligoclase, orthoclase, 
anorthoclase and quartz. 
The zircon is rare and appears in very small stout columnar 
crystals. The iron ore is not abundant, but there is a moderate 
amount of it. The augite is in short, very thick little prisms 
about 0°5"™ long. Prisms and pinacoids are present. It gen- 
erally contains a pale green diopside core which rapidly passes 
into a bright green egirite-augite mantle; often the diopside 
core is wanting and the crystal passes into deep green eegirite 
on the exterior, the optical properties suffering a corresponding 
change. Some examples are changed exteriorly to ferruginous 
products by alteration. The oligoclase appears in the form of 
idiomorphic phenocrysts of tabular form or somewhat columnar 
on the 4 axis about 1™™ long. Both the Carlsbad and albite 
twinnings are present. It is a rather common phenocryst. 
Much more abundant, however, are larger, ill-formed ortho- 
clase crystals which present no unusual features. They com- 
monly show the Carlsbad twin, but a Baveno twin was seen in 
one case. The anorthoclase is also quite abundant and is fre- 
quently intergrown with orthoclase. It contains successive 
rows or sets of short, excessively fine lamelle of albite, twinned 
according to the albite law. It is sharply differentiated from 
the oligoclase both by the difference in double refraction and 
by its method of twinning. All three of these feldspars are 
apt to be collected together into grouped masses. They are 
slightly kaolinized. 
The above phenocrysts lie scattered in a rather fine ground- 
mass composed of allotriomorphic feldspar and quartz. The 
structure is micro-granitic and the amount of quartz is con- 
siderable. The feldspar is mostly alkali feldspar, unstriated, 
but a certain amount of oligoclase is present in the second 
generation. Small microlites of a mineral of strong refraction 
are present which may be of augite. 
The systematic position of this rock type is rather peculiar ; 
it is clearly of high silica, alumina and alkalies, and of these 
latter soda is very largely present. It contains considerable 
iron, while lime and especially magnesia play but an incon- 
spicuous role. It is closely allied to the granite porphyries by 
the large amount of quartz in the groundmass, but it does not 
contain any quartz phenocrysts. On the other hand, the 
amount of plagioclase present, while not sufficient to throw the 
rock out of the alkali group, shows tendencies towards diorite 
porphyrite. Under these conditions it seems most closely 
allied to the syenitic group of rocks and the name of quartz 
syenite porphyry is given it. 
