446 KEMP AND BILLINGSLEY SWEET GRASS HILLS, MONTANA 



central area Colorado shales, which, with numerous sills, still forms the 

 surface. Its profile, when viewed from the northwest, is shown in figure 

 1 of plate 9. The outstanding feature is the sharp cone, illustrated also 

 by figure 2 of plate 9 and figure 1 of plate 10. This cone is the remnant 

 of a laccolith which probably was once more domelike in outline. It 

 rests, however, on Colorado shales, as shown in the section, figure 2, and, 

 as did the intrusive masses of West Butte, the uprising molten rock 

 spread laterally in these relatively soft shales. Several large sills came 

 out much lower in the Colorado formation. The other laccolithic ex- 

 posures, M. 2 and M. 3, are very much lower in altitude, but the general 

 character of the great doming uplift gives ground for inferring a very 

 large laccolith, presumably with its base on the Madison limestone and 

 spreading laterally in the Kootenai shales and sandstones. The many 

 sills of the central area support this view, and it has been adopted in the 

 section of Middle Butte in figure 2. 



- East Butte is the largest of the three uj)lifts and is shown in profile, as 

 viewed from the west, in figure 2 of plate 12. It has several quite lofty 

 and impressive summits, which in the past have received separate names. 

 The bases of the laccoliths rest on the Madison limestone, so far as our 

 observations go, and the igneous magma spread laterally in the Kootenai 

 shales and sandstones. E. 1 and E. 2 of figure 5, while separate summits, 

 are apparently parts of one intrusive mass. E. 3 stands up in a strongly 

 marked cone, as shown in figure 2 of plate 12. E. 6 is also a striking 

 summit, as brought out by the same view. One narrow, continuous ex- 

 posure of igneous rock connects E. 3 and E. 6. It may be that they are 

 residues of one parent mass. The long, straight, northwest boundary of 

 exposure E. 4 suggests a fault, and the relations of it to the other ex- 

 posures is thereby obscured. E. 5 would appear to be a small outlying 

 laccolith in the Colorado shales 2 miles east of the main exposures. Hay- 

 stack Butte is another at the same horizon and 4 or 5 miles to the south- 

 west. Numerous sills, many of them minette, surround the central area 

 and are especially well exposed in the valley which heads up south of the 

 Strode ranch. The minette dike, 4 or 5 miles to the northeast, is the 

 remotest outlying dike of which we have knowledge around the Hills. 



WEST BUTTE 



Speaking in general summary of AVest Butte, the rocks constituting 

 the main laccoliths and the sills are all porphyritic or felsitic in texture. 

 Typical granitoid textures we have not met. The feldspars are the pre- 

 dominant and may rarely be the only, phenocrysts, but there are frequent 

 instances in which the ferromagnesian (mafic) minerals appear promi- 



