138 Weed and Firsson—Bearpaw Mountains, Montam^a. 



The rock is a light-colored trachyte carrying included frag- 

 ments of minette, the weathered surface being pitted with the 

 rusty cavities left by the decomposition and weathering out of 

 these micaceous inclusions, giving the rock a porous appear- 

 ance. It breaks into rather small debris, the pieces averaging 

 5 to 6 inches across. The exposure is but a few hundred 

 yards in diameter and the rock is surrounded by a narrow con- 

 tact ring of indurated, altered sedimentary rocks. The intru- 

 sive mass is covered by these hornstones, the ridge above the 

 postoffice showing occasional detached parts of the cover rest- 

 ing upon the massive igneous rock. It is evident that the 

 intrusive is laccolithic in character, or at least has uplifted and 

 tilted the sediments into which it has been intruded. A 

 hundred feet up the slope the igneous rock holds included 

 fragments of baked shale, much twisted and warped, and the 

 porphyry itself shows slickenside surfaces indicating some 

 movement after consolidation. 



In the hand specimen the rock has a brownish gray color 

 and a rough trachytic feel. It appears plainly holocrystalline 

 to the eye, showing a fine granular groundmass that is 

 sprinkled with numerous phenocrysts. The fine groundmass 

 is seen to be chiefly feldspathic, but it is thickly peppered 

 throughout with very small leaves of biotite which grade to 

 microscopic dimensions and which give the rock its brownish 

 tone. ]No dark mineral appears as a prominent phenocryst. 

 Yery thickly scattered through this groundmass are the phen- 

 ocrysts of feldspar, which is clear, glassy, and of the sanidine 

 habit; it is usually developed columnar on the a axis though 

 sometimes short, thick tabular parallel to the 5(010) face and 

 reaches dimensions of 1^"" in length. It is quite idiomorphic 

 and bounded by the m(llO), 5(010), and c(OOl) faces. 



Besides these phenocrysts the rock is quite thickly dotted 

 with inclusions, angular or subangular in shape, of what appear 

 to be a basic rock, and also of shale fragments brought up 

 from below. They are rather small in size, not often reaching 

 -j^cm j^j^ diameter, and they appear to have been subjected to 

 intense endogenic metamorphism. Some are dense and black, 

 while others are rich in biotite and appear like inclusions of 

 minette. Their former nature can only be surmised, as they 

 are altered and usually converted into masses of limonite, etc., 

 though the containing rock is quite fresh. Some of them 

 present sections which recall resorbed olivines or hornblendes. 



In thin section the microscope discloses the presence of the 

 following minerals : Apatite^ titanite, iron ore^ hornblende, 

 augite, hiotite, alkali feldspar, quartz, and some calcite and 

 decomposition products. 



The IduYgQ feldspar phenocrysts are of orthoclase. This is 



