444 KEMP AND BILLINGSLEY SWEET GRASS HILLS, MONTANA 



of the components. The feldspar phenocrysts were 2 millimeters across. 

 Tire minerals observed and recorded are apatite, hornblende, iron ore, 

 plagioclase, ortlioclase, and quartz. The plagioclase ranges from basic 

 labradorite to medium acid oligoclase. The hornl)lende is the usual dark 

 green color and is more or less decomposed. The groundmass is a fine- 

 grained, patchy or micropoikilitic mixture of quartz and feldspar, the 

 latter usually too much altered or kaolinized for identification, but cer- 

 tainly composed in part of ortlioclase. The quartz occurs also, at times, 

 in irregular grains, which rise to the position of small phenocrysts. They 

 therefore named the rock quartz diorite porphyrite. 



Two specimens were also studied from East Butte. They are described 

 as porphyritic, but as possessed of larger phenocrysts of flesh-colored 

 feldspar (11 x 6 millimeters) than the specimens from West Butte. 

 There are also small black augites. Under the microscope, zircon, iron 

 ore, apatite, segirine-augite, oligoclase, orthoclase, anorthoclase, and 

 quartz can be recognized. The commonest phenocryst is orthoclase, fre- 

 quently intergrown with anorthoclase. The groundmass consists of 

 microgranitic alkali feldspar and quartz, with some small microlites 

 which were suspected of being augite. While the obvious soda-rich char- 

 acter of the rock was recognized from the anorthoclase and segirite- 

 augite, the name quartz syenite porphyry was given to it. As the subse- 

 quent details will show, our much more extensive and representative 

 collections fully bear out the soda-rich character of East Butte and have 

 added the characteristic aegirite rocks, the tinguaites, as well as more 

 acidic members. 



Weed and Pirsson also describe as minette the dike rock from the plains 

 some miles north of East Butte, which Dr. Dawson mentioned twenty 

 years earlier as kersanton. The former found abundant phenocrysts of 

 biotite up to 5 millimeters. Under the microscope, apatite, iron ore, 

 biotite, augite, orthoclase, and calcite were recorded. The augite was 

 sometimes associated with ^girite. This is really a remarkably interest- 

 ing rock, which we also have studied. We find, however, additional sills 

 and dikes of it, both in Middle Butte and East Butte, and some varia- 

 tions toward rocks with biotite still prominent, but not predominant. 



The above earlier records have been cited in detail both to give proper 

 acknowledgment to previous workers and to prevent unnecessary repeti- 

 tion. Laccoliths of rocks which show affinities with the trachyte-syenite 

 series and the andesite-diorite series are widespread in the Eocky Moun- 

 tains. They are frequently mentioned by Weed and Pirsson, but the 

 ones in the Judith Moimtains and in the Moccasin Buttes, lying east of 

 them, are marked in their resemblance to the Sweet Grass types. The 



