458 KEMP AND B1LL1NGSLP:Y SWP:F7r (4RASS HILJvS, MONTANA 



These analyses have many points of similarity. The one from East 

 Butte is about 2 per cent higher in silica than the others, but analyses of 

 phonolitic rocks can be easily found which go higher yet in silica. The 

 ICast Butte rock is the richest in alkalies of the three. Dr. Washington 

 recast the analysis so as to make the rock's formula in the quantitative 

 system II, 5(6), 1.3" and the norm: 



Orthoclase 48 . 93 Diopside 7 . .36 



Albite 16.24 Wollastonite 0.98 



Nephelite 7 . 10 Ilmenite 1 . 06 



Acmite 13.40 Apatite 0..S4 



Sodium metasilicate .... 4 . 76 



Total.* 100.12 



Minette. — Among the, most interesting of all the rocks in the Sweet 

 Grass Hills are the rare rocks, minettes, which appear in sills and dikes. 

 One sill has already been cited from Middle Butte, where there are one 

 or two others. Several sills were mapped in the valley leading south 

 toward East Butte from the Strode ranch. Of these number 46 has been 

 selected as the best case for description and for the analysis, kindly made 

 by Dr. H. S. Washington. The rock forms a sill in the Colorado shales 

 and is cut across and beautifully exposed by the creek valley. It is dark 

 gray in color and richly spangled with shining, black phenocrysts of 

 biotite, up to a centimeter and more in diameter. In almost, but not 

 quite, all cases the biotites lie parallel with the walls and give to the 

 minette the characteristic resemblance to a biotite schist. In thin-section 

 the most impressive mineral is the biotite, which is a rich golden brown 

 color. It is very often in well bounded, six-sided platy-crystals. Some 

 slight bendings from flowage may be detected. Augite is far less abun- 

 dant and in much smaller crystals. It is pale green and has the usual 

 high extinction of common augite. There are no feldspar phenocrysts. 

 Magnetite grains are richly scattered through the rock. As is so often 

 the case with the basic, biotite-rich rocks, apatite is abundant and of 

 relatively large size. The cross-sections indicate stocky hexagonal prisms 

 up to .2 millimeter in height and diameter. One titanite of rather large 

 size for this mineral was detected. In the groundmass rods of an un- 

 twinned, colorless mineral, believed to be orthoclase, may be identified, 

 but no plagioclase was detected. As is such a frequent experience with 

 minettes, there is a large amount of secondary carbonates, and there is 

 also a rather frequent development of clear, apparently secondary quartz. 

 Green chloritic alteration products are not lacking. Dr. Washington's 

 analysis resulted as is given under I. Under II and III are other 

 minettes from the laccolithic mountains of Montana. 



