PETROLOGY IN AMERICA. 467 



very abundant. The other, described as quartz-tin<;uaite 

 porphyry, corresponds very closely with Brogger's grorud- 

 ite, differing chiefly in the presence of large porphyritic 

 crystals of orthoclase. Besides a variety of intrusive rocks, 

 there occur in the Bear Paw Mountains a series of basalts, 

 probably leucitic, but of these no description has yet 

 appeared. 



In the Highwood Mountains (11) in central Montana 

 similar basaltic tuffs and flows occur, while distinct volcanic 

 cores are seen breaking" through the level Cretaceous strata, 

 which are further cut by a large number of dykes with a 

 radial disposition. The most interesting feature, however, 

 is the laccolitic intrusion which forms Square Butte, an 

 isolated mountain some three or four miles in diameter, and 

 rising about 2500 feet above the surrounding plateau. 

 The laccolitic character of the mass is clearly demonstrated, 

 and its structure has been laid bare by erosion. It consists 

 of two types of rock, an inner portion of an acid felspathic 

 type surrounded by a zone of a basic augitic one. The 

 former is a sodalite-syenite consisting of sixty-six parts of 

 felspar, twenty-three of hornblende, eight of sodalite ; and 

 three of analcime : it has already been described by Lind- 

 gren and Melville. The dark type is a syenitic rock, in 

 that its dominant felspar is orthoclase, but it constitutes a 

 peculiar basic variety to which the name shonkinite is given. 

 Augite, containing; 1 per cent, of soda, makes up about half 

 the rock, olivine and iron-ore occur in smaller amounts, 

 while apatite, sodalite, nepheline, etc., are accessories. 

 Chemical analyses of the two rocks show a notable difference, 

 the former being richer in silica, alumina and alkalies, the 

 latter in iron-oxides, magnesia, and lime. Examination 

 proves that they are parts of a single intrusive body of 

 rock, though the transition from one to the other is a rapid 

 one. The phenomena seem to show conclusively that the 

 magma was injected in a homogenous condition, and was 

 differentiated in place. That the differentiation was, at 

 least in part, effected prior to crystallisation appears from 

 the difference between the ferro-magnesian minerals of the 

 two rocks. 



