SOME PETROGRAPHIC DETAILS 457 



orthoclase. Multiple twinning fails, but the crystals are inclined to de- 

 velop partially altered centers, giving them a muddy aspect. The dark 

 phenocrysts are beautiful yellow-green segirites or segirite-augites, with 

 very excellent crystal outlines, twins and zonal structure. Pleochroism 

 ranges from yellowish green to yellowish brown. The groundmass is the 

 usual felt of fine green segirite needles, showing fiowage arrangement and 

 set in an isotropic or very feebly refracting basis. The whole aspect is 

 the characteristic one of the tinguaites. One can not resist the belief 

 that nephelite is present, or at least analcite, but gelatinizing tests with 

 the pulverized rock, both by the senior writer and by H. S. Washington, 

 when making the analysis given below, failed to afford satisfactory re- 

 sults. The combined water is so low, 0.45, that the amount of possible 

 •analcite would be small. The analysis shows the silica to be rather high 

 for good nephelite or analcite rocks, while at the same time one can not 

 well recast it without using one molecule or the other. Analyses of two 

 other American rocks of related character are placed beside it, both going 

 even slightly higher in silica and also in alumina, but lower in the other 

 bases. The suggestion may be made that, as we are dealing with a pipe 

 only 10 feet in diameter, the uncrystallized residue may have chilled into 

 a glass which, had it broken up into minerals, would have afforded both 

 feldspars and nephelite or analcite. No quartz whatever has been detected 



in the rock. 



I II III 



SiO, 59. .39 57.46 57.63 



TiO, .' 0.58 .60 .23 



A1X)3 14.65 15.40 17.53 



FeJ), 4.58 4.87 3.46 



FeO 1.40 .87 1.18 



MnO 0.13 tr. tr. 



MgO 0.70 1.37 .22 



CaO 2.39 2.59 1.35 



Na,0 7.68 5.48 5.80 



K,0 8.25 9.44 9.16 



H=0-^ 0.45 .82) ^^ 



H,0— 0.11 .09( '^•-- 



P.O-, 0.13 .31 



. 100.44 99.20 99.78 



I. Tiiiguaite pipe. East Butte, Sweet (iiass Hills. Specimen 125. Analysis 

 by H. S. Washington for this contribution. 

 II. Tinguaite dike, Bean Creek, Bearpaw Mountains, Montana. Weed and 

 Pirsson. American Journal of Science, 1896, volume II, page 192. H. N. 

 Stokes, analyst. Also CI, .20; SO3, 13; CO,, .13; BaO. 60; SrO, .16; 

 making a total of 100.42. 

 HI. Tinguaite dike. Cone Butte, Judith Mountains, Montana. As under II. 

 li. V. I'irsson, analyst. Also CI, .08; making a total of 99.S6. 



