SOME PETROGRAPHIC DETAILS 459 



I II III 



SiOi. 49.98 52.26 46.04 



TiOa 1.24 .58 .64 



AI2O3 12.91 13.96 12.23 



Fe^Oa 2.49 2.76 3.86 



FeO 4.21 4.45 4.60 



MnO 17 .14 



MgO 7.03 8.21 10.38 



CaO 7.65 7.06 8.97 



Na^O 2.64 2.80 2.42 



K,0 5.02 3.87 5.77 



H2O+ 1.78 1.34 2.87 



H2O— 57 1.53 



CO2 2.84 .49 



P2O5 86 .52 1.14 



BaO 43 .23 .48 



SrO not det. .05 .25 



SO3 09 not det. tr. 



99.91 100.25 99.65 



I. Minette, East Butte, Sweet Grass Hills. H. S. Washington, analyst. 

 II. Augite-minette, Sheep Creek, Little Belt Mountains, Montana. L. V. 

 Pirsson, U. S. Geological Survey, 20th Annual Report, Part III, 1900, 

 page 531. W. F. Hillebrand, analyst. 

 III. Minette Arrow Peak, Highwood Mountains, Montana. L. V. Pirsson, U. S. 

 Geological Survey, Bulletin 237, 1905, page 145. H. W. Foote, analyst. 

 Also CI, 11 ; total, 99.76. 



In the East Butte minette, when one endeavors to recast the analj'ses, 

 the 3.84 COo and the 1.78 combmed water reveal such advanced altera- 

 tion as to make the determination of either mode or norm a matter of 

 uncertainty. One only needs, however, to compare and study the analyses 

 in order to realize the close similarities which prevail among the several 

 occurrences. The descriptions corroborate the conclusions from the 

 chemical analyses. The same minerals, the same textures, and the same 

 alteration products appear in all three cases. 



Along the valley to the south of number 40 are other sills and some 

 dikes, among which minettes appear not infrequently. Number 49, Just 

 l)eyond number 48 of the map, is a variant from the typical variety. 

 There is much less biotite and, under the microscope, quite as much 

 augite as biotite. Hornblende does not fail, nor is plagioelase altogether 

 lacking. The small minerals — magnetite, apatite, and microlites of 

 augite — are important in the groundmass. The sill contains many inclu- 

 sions of granite and some quartzite, both presumably Precambrian. 

 Tliere are also inclusions of the types of rocks found in the laccoliths, 

 proving the sill to be a later outbreak tliaii are they. 



