450 KEMP AND BILLINGSLEY SWEET GRASS HILLS, MONTANA 



elusion which is now a granular aggregate of pale green augite, shreds of 

 brown biotite, plagioclase, a little orthoclase showing strains, and mag- 

 netite. A reaction rim of radiating hornblende needles borders the inclu- 

 sion. The inclusion might have once been a calcareous shale. 



Sills do not fail around West Butte, but are by no means as prominent 

 as they are around Middle and East buttes. They present no marked 

 differences from the laccolithic rocks already described, except that in 

 specimen number 8, as located on figure 3, we noted purple fluorite. The 

 sills in this vicinity have a greenish color, which led us to expect phono- 

 lites or tinguaites, but the slides showed that the color was due to chlo- 

 rite, and no rich soda-bearing types were identified until we reached East 

 Butte. 



MIDDLE BUTTE 



Middle Butte is marked by the conical erosion remnant of a once 

 larger laccolith, which crowns the south end of the general upheaval. In 

 the central - portion of the doming upheaval the Colorado formation is 

 exposed, with quaquaversal dips and one outstanding top of a laccolith, 

 M. 2 of the map, figure 4. The Colorado sediments are so richly traversed 

 by radiating sills and dikes as to justify the inference of an intrusive 

 mass beneath them. Presumably either the spreading lower part of M. 2 

 is there or a more deeply seated additional laccolith. At the northeast 

 corner is an extended exposure, labeled M. 3 and M. 4, in two summits, 

 which may be eroded portions of one parent laccolith. The rock of M. 1 

 is predominantly or almost exclusively a plagioclase-bearing variety. It 

 is not so strikingly porphyritic in its central mass as are the dikes and 

 sills. The phenocrysts rarely reach 4 millimeters, and thus are not so 

 contrasted with the general grain as to prevent the impression of an al- 

 most granitoid texture. They are chiefly plagioclase, which ranges from 

 andesine througli labradorite. A few zonal orthoclases are associated. 

 C!ommon augite is the predominant ferromagnesian (mafic) mineral. 

 Brown hornblende is rare. Magnetite is abundant. The groundmass 

 consists of rods and stocky rectangles afforded by the cross-sections of 

 plagioclases. With them are mingled irregularly shaped augites, and 

 both display well developed fiow-lines. The rock is of simple mineralogy 

 and is reminiscent of diabases. The name augite diorite porphyry would 

 best describe it, since orthoclase is hardly abundant enough to place it 

 with the monzonites. 



Along the under contact the texture is more decidedly porphyritic and 

 the hornblende and augite phenocrysts surpass those of feldspar. The 

 texture of the groundmass varies from microgranitic to andesitic. Much 

 calcite is aft'orded l)y alteration. The contrasted plienocrysts — feldspars 



