456 KEMP AND BILLINGSLEY SWEET GRASS HILLS, MONTANA 



have been found which followed the diagonals of the square, suggesting 

 that the crystals were bounded by the unit cli no-dome and elongated on 

 the a-axis. The quartz is subordinate and of irregular outlines. Its 

 presence eliminates the possibility of nephelite in the rock. The oegirite 

 forms the usual felty, interlacing mass of minute green needles. An 

 occasional grain of magnetite may be detected. 



These rocks recall at once the soda-rich varieties studied liy W. 0. 

 Brogger in the vicinity of Christiania and described in the classic mono- 

 graph on the "Minerals of the Syenite-pegmatite Dikes." ^^ Grorudite 

 was applied to a dike rock whose phenocrysts were microline and segirite 

 and whose groundmass consisted of rectangular orthoclase, quartz, and 

 ffigirite. We do not find microline in the Sweet Grass rocks. The albite 

 molecule has presumably remained in solid solution in the orthoclase^^ 

 and has not exercised its influence either in the development of microline 

 or microperthite. Aside from this difference, the rocks just described 

 are obviously closely akin to grorudites. 



In the irregularly shaped sill or partly exposed laccolith E. 4, the 

 ffigirite fails, so far as our three slides give the data. The rock is rather 

 coarsely porphyritic, with abundant phenocrysts of orthoclase. Both 

 augite and hornblende accompany it. The hornblende is a beautiful dark 

 green to brown variety, in six-sided crystals with alm.ost parallel extinc- 

 tion. The rock is rich in titanite, a rather infrequent mineral in the 

 other salic types. The groundmass is composed of square and rectangular 

 orthoclases, or at least untwinned feldspar, making the rock a very good 

 syenite porphyry. IV^agnetite is also a small component. 



SOME PETROGRAPHIC DETAILS 



Tinguaite. — Just north of this last exposure, and penetrating the 

 Madison limestone in the brook valley, is the plug of green rock, number 

 135. It is strongly porphyritic, having abundant shining, white, rectan- 

 gular feldspars, 5 to 15 millimeters in long dimension, set in a dense 

 green groundmass. Shining, black prisms of some ferromagnesian min- 

 eral, 0.5 millimeter wide by a maximum of 2 millimeters long, can also 

 be detected. 



Under the microscope the feldspar phenocrysts are apparently all 



1" W. C. Brogger : Zeitschrift fvir Krystallographle, vol. xvi. 1800. On grorudite, p. (ir>. 

 On solvsbergite, Eruptivgesteine des Krlstianiagebietes, I. 1894, p. 67. Solvsbergite has 

 much less quartz than grorudite or none at all. In other respects the mineralogy is 

 much the same, involving alkali feldspar (mostly albite and microcline) with a?girite. 

 In more basic varieties, hornblende (kataforite) replaces fegirite, with sometimes a 

 peculiar mica. When quartz fails altogether, nephelite may appear, 



" The feldspars are discussed from the standpoint of physical chemistry and the con- 

 ceptions used in metallographic studies by H, L. Ailing in vol, xxix, no. 3, 1021, of the 

 .Tournal of Geology. 



