and other Rocks from Montana. 289 



Mr. J. E. Wolff,* but the petrographical character of the two 

 volcanic districts is somewhat different. 



The intrusive rocks of the Highwood Mountains are in 

 general basic in composition and holocrystalline in structure. 

 There are a few coarsely granular rocks, principally dikes of 

 augite syenites, but a much more common type of rock is of a 

 porphyritic structure and closely allied to the trachytes. f 



Augite- trachytes aie frequent, usually containing two gene- 

 rations both of the orthoclase and the augite. The latter is 

 characterized by a deep green color and evidently contains an 

 admixture of the acmite molecule. The quantity of augite in 

 these rocks is sometimes very large, and they grade over into 

 basaltic rocks with orthoclase, plagioclase, olivine and augite. 



Attention should be called to the great similarity of these 

 rocks with the peculiar intrusive and extrusive masses recently 

 described by Mr. Iddings from the Crandall basin and the 

 Absaroka range in the National Park region.;}: 



Another interesting type in the Highwood Mountains is that 

 of the analcite-basalts which, with holocrystalline porphyritic 

 structure, consist of augite, olivine, brown mica and analcite. 

 The latter mineral here appears under conditions strongly sug- 

 gesting a primary origin. 



Square Butte and vicinity was not visited by me in 1883, and 

 1 am again indebted to Dr. C. A. White for notes regarding it. 

 As already mentioned, it forms the extreme eastern part of the 

 Highwood Mountains with which it is connected by several 

 lower buttes and ridges. The elevation of the fiat top, about 

 three-quarters of a mile in diameter, is 5600 feet above sea 

 level according to the maps of the Northern Transcontinental 

 Survey (Fort Benton sheet, U. S. Geol. Survey). The Butte 

 is composed of a light gray eruptive rock with very distinct 

 lamination. In the elevated table land surrounding its base 

 are found several horizontal sheets of a dark gray or black 

 volcanic rock interbedded with black Cretaceous shale be- 

 longing to the Fort Benton group. There are three distinct 

 sheets of this dark rock, each about eight feet thick and 

 separated by beds of shale of about the same thickness. 

 Surrounding Square Butte there are numerous dikes apparently 

 radiating from the central mass. 



The dark volcanic sheets are represented in the collection by 

 three types. Unfortunately, most of them are very much 

 decomposed. The first is porphyritic and contains as ph.en.o- 

 crysts augite of the Highwood type, olivine, usually brown 



*Bull. Geol. Soc. Amer., vol. iii, p. 445. 

 \ This term is here used without restriction to surface flows. 

 % The Origin of Igneous Rocks, by J. P. Iddings. Phil. Soc, Washington, 

 D. C, vol. xii, p. 169. 



