. Bulletin of the Geological Society of America. Geology. 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
. Bulletin of the Geological Society of America. Geology. 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 minetteâ half sill, half dikeâwhich cuts the lower exposure of shale and feathers. :- porphur !j 7^ E ' ' "^ Li'oht-colore-d Si t icafes IHooicna I .Shafts 6^£ SSt4 X, X. " X -irâ K K y it y Si M Kootenai Shale 5 . pA^x/vz-v^ Faulred SiJI ^V^^"* of Mmette '^ L â *- Figure 6.âTypes 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 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 Please note that the
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectgeology, bookyear1890
