4G0 KEMP AND BILLINGSLEY SWEET GRASS HILLS, MONTANA 



In this valley are various other types of rocks in the dikes and sills, 

 among which the slides show a wide range from trachytic mixtures of 

 hornblende and orthoclase, through variations with augite, biotite, and 

 plagioclase, one or several, in different relative amounts. Once one \\n- 

 derstands the variations, detailed descriptions and elaborate nomenclature 

 become unduly tedious. 



In an exposure at number 52, of figure 5, on the west side of the valley, 

 the intersecting dikes and sills illustrated in figure 6 were sketched. 

 They are igneous rocks of at least two ages. Kootenai shales and a bed 

 of sandstone are the wall rocks. Apparently the oldest rock is a minette — 

 half sill, half dike — which cuts the lower exposure of shale and feathers 



:- porphur !j 

 7^ E ' ' 



"^ Li'oht-colore-d 



Si t i cafes 



IHooicna I 



.Shafts 



6^£ SSt4 

 X, X. " X -ir— 

 K K y it y KMtrte.TTe 



Si M 



Kootenai 

 Shale 5 . 



pA^x/vz-v^ Faulred SiJI 

 ^V^^"* of Mmette 



'^ L ■*- 



Figure 6. — Types of Dikes and 8ills 



out at the overlying sandstone. It is twice faulted in a small way, a fea- 

 ture exhibited by some of the other igneous masses. A sill, also of 

 minette, lies above the sandstone bed, but presents some microscopic 

 features unlike any other rock studied from the Hills. The usual large 

 crystals of biotite are quite richly developed. In the groundmass, how- 

 ever, rods of orthoclase up to 0.3 millimeter are arranged in radiating 

 rosettes and starlike aggregates. Other rods exhibit flow-lines. Apatite 

 is strikingly abundant. Masses of carbonates still preserve the outlines 

 of augites. Pyrite is not infrequent. At the top of the series is a light- 

 colored sill, of much more feldspathic rock than the others, but it has not 

 been examined microscopically. All the sills are cut by a dike of biotite 

 porphyry, meaning by the term a trachytic rock. The scale is in feet. 



