448 KEMr AND BILLINGSLEY SWEET GRASS HILLS, MONTANA 



amount. There is one locality, specimen 118, on the southwest corner of 

 West Butte, Avhere the orthoclases reach a full inch in diameter and pre- 

 dominate in the rock. The plagioclases, in our experience and as stated 

 by Weed and Pirsson, range from oligoclase to basic labradorite. Both 

 orthoclase and plagioclase are beautifully zonal in many instances. 



Tlie ferromagnesian (mafic) mineral which may be taken as charac- 

 teristic of the type is hornblende. It may be practically the only one 

 present. There is, however, a constant tendency of augite and biotite to 

 join it and for augite to exceed it. The augite is commonly the ordinary 

 green variety, but, by increasing pleochroism and by the characteristic 

 emerald green color, the presence of the segirite molecule is suggested at 

 times. Augite may become so abundant, combined with increase of basic 

 plagioclase, as to give the rock a basaltic aspect and to remind one of 

 latites and even of olivine-free diabases, but we have never detected oli- 

 vine in the slides. The varieties may be considered merely basic extremes 

 of the usual laccolithic rock. We have found no exposure of rock corre- 

 sponding to the dark, basic yogoite of Weed and Pirsson. 



The hornblende and the less common biotite of the thin-sections show 

 resorption to partial or even complete reorganization, with the produc- 

 tion of much finely granular magnetite. Augite has escaped the resorp- 

 tion and at most has narrow rims accentuated by grains of included 

 magnetite. From the summit of laccolith W. 1 we have one slide, number 

 121, which has a small fragment of an older and more basic, biotite-bear- 

 ing variety included in the later feldspathic porphyry. The inclusion is 

 a reminder of an earlier crystallization of the magma. 



The slides display decided variety in the groundmass, even from neigh- 

 boring parts of the same laccolith. We find microgranitic texture in 

 some. Others have rods of feldspars in recognizable flow-structures, re- 

 minding one of typical trachytic texture. The feldspars may be inter- 

 laced and fairly coarse rods of plagioclase, reminiscent of diabases, and 

 may again assume stocky, rectangular shapes, almost square. From the 

 east side of laccolith W. 2, specimen 116, and from the summit, specimen 

 117, have the stocky feldspars (plagioclase in these instances) in rude 

 flow-lines. The latter has so much augite as to make a basaltic impres- 

 sion on the observer. 



We have reviewed the laccolithic rocks of West Butte in detail because 

 they furnish the most accessible collecting ground and are, on the whole, 

 the best exposed. Along the precipitous front on the southeast corner, 

 there is a huge talus of beautifully fresh rock which furnishes a very 

 interesting exhibit and to which a visitor may resort with the best oppor- 

 tunity for hand specimens. From the talus (number 1 of figure 3) was 



