288 Lindgren and Melville — Sodalite-Syenite 



meters long and flakes of a brown, slightly pleochroic mica 

 with very small axial angle. The grounclmass consists mostly 

 of lath-shaped plagioclase crystals together with augite and 

 mica. In some specimens with phenocrysts of olivine and 

 angite the gronndmass is glassy and contains no feldspar ; this 

 rock approaches closely to certain limburgites. The general 

 appearance of the Bear Paw series and the absence of pheno- 

 crysts of feldspar in it points to its connection with the 

 group which Rosenbusch has called the lamprophyric dike 

 rocks. 



The suite of specimens collected at Square Butte is of par- 

 ticular interest. Square Butte, which really forms the eastern 

 end of the Highwood Mountains, is situated thirty miles 

 southeast of Fort Benton and eighteen miles nearly due east 

 of Highwood Peak. The rocks from this locality show a 

 close relationship with those from the main group of the 

 Highwood Mountains, and it may perhaps not be amiss to 

 refer briefly to the character and rock types of the latter.* 



The Highwood Mountains with their sharp and jagged peaks 

 and ridges stand in isolated grandeur on the monotonous 

 plains twenty miles south of Port Benton on the Missouri 

 river. They form an oblong group twenty miles from north 

 to south and thirty miles from east to west and their highest 

 peaks rise 3600 feet above the surrounding Cretaceous plateau 

 which here consists of the nearly horizontal black shales of 

 the Fort Benton group. The mass of the mountains is made 

 up of a network of dikes and probably also of laccolitic 

 masses between which are included contact metamorphosed 

 and disturbed remnants of sediments none of which are be- 

 lieved to be older than the Cretaceous. Above the Fort 

 Benton group once rested the whole thickness of the Montana 

 and Laramie formations or at least 8000 feet of sediments. 

 Volcanic activity began at this point at or after the close of the 

 Cretaceous period. Great quantities of igneous rocks were 

 forced into the sediments and on the surface the eruption was 

 probably connected with the phenomena of a subaerial vol- 

 cano. Subsequent erosion has removed nearly the whole 

 thickness of the softer Laramie rocks, exposing the harder core 

 of the ancient volcano and the abyssal rocks solidified under a 

 pressure of many thousand feet of superincumbent sediments. 



The Highwood Mountains are very similar in structure to 

 the Crazy Mountains, also in Montana, recently described by 



* The general geology of the Highwood Mountains has been described by Prof. 

 W. M. Davis in 10th Census, vol. xv, p. 697, and the petrography by W. Lindgren, 

 loc tit., p. 729. See also, " Eruptive Rocks from Montana," by W. Lindgren. 

 Proc. Calif. Acad. Sci., series II, vol. iii, p. 40. 



