452 KEMP AND BlLLIN(jlSLP]Y SWEET GRASS JIILLS, MONTANA 



in the central portions, fcrroniagnesian miuerals at the h)\\'er contact — 

 indicate increasing basicity toward the bottom, but tlie contrasts are not 

 so great as to develop yogoite below. 



Of the other three laccoliths no specimens were gathered, although they 

 have all been visited and mapped. To the eye, they seemed superficially 

 like the larger southern one. 



The dikes and sills present much greater variety than do the laccoliths. 

 Among themselves the two groups display such similar varieties of rocks 

 that there is no advantage in treating them separately. One can not 

 reasonably expect, in associated small bodies of intrusives, any great con- 

 trasts, because some are parallel with the bedding of the sediments and 

 some cut across it. At one extreme there is a type with large pheno- 

 crysts of green hornblende, with no other phenocrysts and, even under 

 the microscope, with no other recognizable ferromagnesian components. 

 The groundmass consists of multitudes of minute feldspar rods, which in 

 the vast majority of cases are once twinned and have nearly or actual 

 parallel extinction. Only a few clearly show multiple twinning. Alter- 

 ation somewhat obscures the clearness of individuals. Flow-lines are 

 well developed. There are some little groups and individuals of quartz. 

 Magnetite and apatite are fairly abundant, and there are a few spots of 

 secondary calcite, which, as the hornblende is notably unaltered, may reiD- 

 resent thoroughly decomposed augites. From this extreme of trachytic 

 character we pass to increasing amounts of augite and plagioclase in 

 other dikes and sills, and to some variations in the texture of the ground- 

 mass, because of larger rods and stocky rectangles among the feldspars. 

 We never find more than a little quartz, but often see much calcite of a 

 secondary character. Biotite first makes its appearance in scattered 

 shreds. On its increase it seems to replace the hornblende, and with the 

 disappearance of the latter fairly pure augite-biotite varieties result. 

 With the further increase of biotite it may be the principal and pre- 

 dominant phenocryst, and in some of the sills reaches a large size, over 

 40 millimeters in diameter. Augite is its favorite associate and ortho- 

 clase is so prominent as to make the rock a minette. 



The most interesting sill of this character is the one numbered 3!) on 

 the map and situated on the south side of the upheaval, a short distance 

 northeast of the Fey ranch. It is illustrated in both pictures of plate 10. 

 It is about 30 feet thick and has separated into marked columns on cool- 

 ing. The abundant biotite first catches the attention in the gray mass 

 afforded by the weathering of a rock which is green when fresh. Under 

 the microscope the biotite is not so prominent, but, while abundant, is 

 associated with equal amounts of large and small augite, sometimes 



