292 Weed and Pirsson — Bearpaw Mountains, Montana. 



where it forms a massively jointed wall extending north and 

 south along its crest, while below large talus blocks cover a 

 steep slope extending down to the top of bold cliffs of the 

 same rock. The cliffs, seen from the surrounding country, 

 form the most noticeable feature of the butte. On three sides 

 they form an almost insurmountable wall, over a hundred feet 

 in height. In these cliffs the syenite-porphyry has a massive, 

 platy structure, the lamination being parallel to the outer sur- 

 face of the cliffs, dipping on all sides away from the butte at 

 30° to 45°. Though the rock sometimes breaks along these 

 planes and forms a smooth rock slope, the exposed face of the 

 cliffs is generally much steeper and corresponds to a jointing 

 of the rock. The debris consists of blocks, often 8 feet in 

 diameter, with very little material less than a foot across. 



The weathered rock has a botryoidal surface that is rather 

 characteristic. No difference is recognizable between the rock 

 forming these cliffs and that of the summit of the butte, either 

 in the nature or relative amounts of the component minerals, 

 or in the granularity or physical structure. It appears prob- 

 able, however, that erosion has as yet only exposed the outer 

 part of the boss, and that both summit and side show only the 

 outer portion of the intruded mass. 



On a surface of fresh fracture the rock is a very pale-gray 

 color mottled with very pale-pink; on a somewhat weathered 

 surface these contrasts of coloring are accentuated. Upon 

 closer examination the rock is seen to be chiefly composed of 

 large, thickly crowded crystals of feldspar, held in a dense 

 pale-green ground-mass that is speckled with small black 

 augite crystals. The pale-pink tone of the rock is due to these 

 thickly crowded phenocrysts of feldspar. The black augites 

 which occur plentifully scattered through the ground-mass 

 rarely attain a length of 2 mm . The feldspar phenocrysts are 

 stout in habit and vary in size from one to one-half centimeter 

 in length. They usually possess the orthoclase habit, are very 

 rarely clear glassy and sanidine-like, and are developed in 

 short, stout, columnar forms on the clino axis ; they are apt to 

 be more or less rounded and anhedral.* When idiomorphic 

 they show the common planes m(110), &(010), and c(001). 

 They are frequently carlsbad, less often baveno twins. A few 

 sporadic grains of quartz have also been observed, though they 

 do not appear to be at all common. 



* A term suggested for rounded formless masses which do not show outward 

 crystal form. See report of the winter meeting of the Geol. Soc. Am. 1895, in its 

 proceedings, also abstracts in this Journal, p. 150, vol. i, 1896; Jour. Geol. vol. 

 iv, 1896, p. 128. It has been suggested that the terms anhedron and anhedral 

 conflict with allotriomorphic ; this is by no means the case, since allotriomorphic 

 is a term of structure denoting a formal relation of parts, while anhedron is indi- 

 vidual and used in a crystallographic sense. 



