Pir 88on — Petrographio Province of Central Montana. 39 



the most salic to the most femic types, the depth of color 

 usually increasing somewhat towards the ferromagnesian pole. 

 It is commonly supposed that the purplish color of augite is 

 due to the titanic oxide it contains ; and while this perhaps is 

 true, it should nevertheless be pointed out that one of these 

 green pyroxenes from the shonkinite of Square Butte, ana- 

 lyzed by the writer, contained over a half per cent of titanic 

 oxide. It is also to be noted that titanic oxide occurs in all of 

 these rocks, gradually increasing with the iron towards the 

 ferromagnesian pole, yet the rocks towards this end still have 

 the strong green color in the pyroxene. This is especially 

 noticeable in the shonkinites of Yogo peak in the Little Belts, 

 in the various occurrences in the Highwoods and in the Beaver 

 stock and elsewhere in the Bearpaws, the Ti0 2 ranging from 

 0*75 to 1'50 per cent, the silica falling as low as 46 per cent in 

 the latter case. The occurrence of this green augite through 

 the whole series is more strongly marked in the Highw T oods 

 than elsewhere and this local peculiarity did not escape Lind- 

 gren's notice and he makes especial mention of it,* not only 

 for the Highwoods but for the other groups of the region 

 with which he was acquainted. There is no notable exception 

 to this rule in any of the Highwood rocks numbering several 

 hundred occurrences studied by the writer, no matter how salic 

 or femic the types may be. 



This green augite is a marked feature then of this petro- 

 graphic province, and in this respect it appears to differ from 

 many other well-marked provinces of alkalic rocks. In the 

 exceptional cases mentioned above, the augite is pale brown, 

 strong purplish colors not having been noted, so far as the 

 writer can recall, in the whole province. 



In the salic rocks rich in alkalies, aegirite-augite appears : 

 this is a marked feature of those of tinguoid habit ; aegirite 

 itself is rare. This seems to be due to the dominance of 

 potash over soda, as will be shown in the discussion of the 

 chemical peculiarities of the province. It is possible that the 

 characters of the pyroxene, including its green color and non- 

 pleochroism, are also due to this general chemical character of 

 the magmas. 



Biotite. — Throughout the province the biotites are the 

 brown, strongly pleochroic variety — ordinary biotite. The red- 

 brown biotites of the theralite rocks found in the Crazy Moun- 

 tains to the southward do not occur, nor the pale phlogopites 

 of the rocks rich in potash of the Leucite Hills in "Wyoming 

 as described by Zirkelf and Cross.;}: In some exceptional 



* 10th Census United States, vol. xv, p. 726, 1886. 

 ■f Micro. Petrog. 40th Parallel Surv., vol. vi, p. 261. 



% Igneous Rocks of the Leucite Hills and Pilot Butte, Wyoming ; this 

 Journal, vol. iv, 1897, p. 120. 



