A Little-Known Region in Northwestern Montana. 197 
hardened by the heat and pressure. The diorite sheet, although in 
some exposures the surface formation and in others hundreds of feet 
below the surface, is nevertheless found at very nearly the same geologi¬ 
cal level in each of the exposures. 
2. Petrographical notes.— The petrographical study of this mass of 
rock has been made on four specimens, three of which are from the lo¬ 
cality marked 1, where a cliff almost forty feet high is capped by a green 
sandy shale. Specimen 1 is from near the bottom of the cliff, perhaps 
thirty feet from the contact. Specimen 2 was taken from a point about 
twenty feet, and specimen 3 about ten feet below the contact, so that the 
specimens show in inverse order the gradations in characters of the 
rock in passing from the contact toward the center of the mass. The 
sedimentary rock of the cap is well baked. The other specimen of the 
eruptive rock which has been sectioned is from the contact with shale at 
locality 3, and shows portions of both rocks in the section. 
The rock is all cases much decomposed. Specimens 1, 2 and 3 show a 
steady decrease in the coarseness of texture, the crystals in No. 1 having 
average dimensions of from one to two millimeters, with occasional 
hornblende crystals sometimes attaining to a half centimeter in diam¬ 
eter. The crystals in No. 2 will average about one-half, and those of 
No. 3 about one-fourth these dimensions. 
All the specimens from locality 1 have a more or less apparent gran¬ 
itic structure with a tendency to porphyritic development of the horn¬ 
blende. A dull greenish lustre so characteristic of rocks of this type is 
apparent in all, but somewhat irregularly distributed. The lens reveals 
in the coarser specimens feldspar, augite, hornblende, black ore material 
and pyrite. When the rock was powdered a considerable portion of the 
ore material was strongly attracted to the magnet, indicating magnetite. 
In addition to the minerals above mentioned the microscope reveals in 
all sections ilmenite, leucoxene, quartz, chlorite, zoisite, and apatite. 
Section 2 contains well crystallized sphene and section 3 epidote as well 
as sphene. 
The more coarsely crystalline of the three shows in section a plagio- 
clase which is very much decomposed and is associated with more or less 
micro-pegmatite. The nature of the alteration seems to be saus- 
suritization, zoisite being identified as one of the products by its moder¬ 
ately high index of refraction, its parallel extinction and low interference 
colors. Augite and hornblende are present in about equal proportions, 
but the crystals of the former are much larger than those of the latter 
The hornblende is the brown variety with deep colors. The absorption 
is c > b > > a. It is always well outlined and shows perfect prismatic 
cleavage. It is very unstable, the initial stages in the alteration consist¬ 
ing apparently in a change of color to green, followed by a decomposi¬ 
tion to chlorite. This decomposition has in some cases proceeded so far 
