408 WEED AND PIRSSON — HIGHWOOD MOUNTAINS OF MONTANA. 



dark stone color, like many basic diorites. In the hand specimen, how- 

 ever, it is found to be so coarse grained that the distinction between the 

 dark colored ferro-magnesian components and the light colored feld- 

 spathic ones becomes strongly accentuated, the contrast giving the rock 

 a mottled appearance. 



Thus by inspection of the specimen one readily distinguishes the chief 

 components. They are augite in well formed, often rather slender, idio- 

 morphic crystals, of a greenish black color, attaining at times a length of 

 one centimeter, but not averaging perhaps more than a quarter of that 

 length, and biotite, of a bronzy brown color, whose occasional cleavage 

 surfaces attain a breadth of from one to two centimeters, but whose out- 

 lines are not clear and idiomorphic, but irreguler, dying away among 

 the other components in shapeless patches. These biotites are, more- 

 over, extremely poikilitic, inclosing the other components. With the 

 lens these broad cleavages are seen to be made up of great numbers of 

 smaller biotite individuals in parallel growths, but including the other 

 minerals. They are thus, as one might say, spongy, skeleton crystals on 

 a large scale. 



Filling the interspaces between these dark minerals is a white feld- 

 spathic material, from which one obtains occasionally the reflection of 

 a good feldspar cleavage. With the lens one detects greenish grains of 

 olivine in addition. 



An inspection of the rock shows at once that its predominant character 

 is the great abundance of the augite, which must form at least one-half 

 of the mass by volume and a greater proportion by weight. AVith this 

 large amount of augite, it is clear that if it, were a dense fine grained rock 

 instead of being so coarse grained as it actually is, a pronounced basaltic 

 appearance would characterize it. 



In texture the rock is rather friable and crumbly, and blows of the 

 hammer will frequently cause a specimen to fall into a coarse gravel. 

 This is not due necessarily to alteration, but to the great number of 

 pyroxene prisms and their idiomorphic character, there being little adhe- 

 sion betwcBn their polished faces and the white feldspar material which 

 fills their interspaces. A single heavy blow will often loosen these prisms 

 so that the rock will crumble under the fingers. 



In thin-sections under the microscope the following minerals are found 

 to be present : Apatite, iron ore, olivine, biotite, augite, albite, anorthoclase, 

 orthoclase, sodalite, nephelite (?), cancrinite (?) and zeolites. 



Apatite. — This is the oldest mineral appearing in idiomorphic outlines 

 even in or abutting into the iron ore. It is in short, stout prisms which 

 often attain a length of 0.5 millimeter. Though commonly colorless, it 

 is at times filled with excessively fine, dusty particles, and then becomes 



