194 Weed and Pirsson-^Bearpaw Mou7itains of Montana. 



lime, magnesia, and ferrous iron are exactly balanced for the 

 pyroxene molecule, which is here called diopside. 



Quartz-tinguaite-porphyry . — West of the Wind Butte core 

 the hardened shales of the contact zone are cut by numerous 

 dikes. Several of these consist of a minette-like type which 

 disintegrates rapidly, so that the presence of the dike can only 

 be recognized by the nature of the soil. Prospect pits have 

 been sunk upon a dike of this micaceous rock, which is locally 

 decomposed, of a white or yellowish or rusty color, and is said 

 by miners to have an assay value of $4 per ton. The other 

 dikes are green porphyritic tinguaitic rocks of varying types. 

 The usual variety is a quartz-tinguaite-porjphyry . A fresh 

 fracture shows a dense tough ground mass of a clear light- 

 green color, in which are imbedded small square phenocrysts 

 of white feldspar. The latter are equidimensional and average 

 1 to 2°""" in diameter ; they are so thickly crowded that there 

 appears almost as much feldspar as groundmass. A few of 

 them are much larger than the average and are of tabular 

 habit. With the lens a very few, minute, black augites can be 

 seen in the groundmass. 



The study of the thin section shows the rock to be of very 

 simple composition, soda-orthoclase and segirite-augite pheno- 

 crysts lying in a groundmass of alkali feldspar, quartz, and 

 ?egirite microlites. The feldspar phenocrysts have the mottled 

 moire appearance and the albite microlites as inclusions 

 described in detail in our former paper under the syenite- 

 porphyry of Gray Butte ; they have been studied, but demand 

 no further notice. The aegirite-augite phenocrysts are rather 

 deficient in the segirite molecule, otherwise they are of the 

 usual type. The groundmass, which is of extremely fine grain, 

 is thoroughly allotriomorphic in structure and the amount of 

 quartz is considerable ; the segirite is scattered through it in 

 such great quantities, as slender needle-like microlites, that the 

 section has a mossy appearance. 



The rock in thin section very closely resembles the gro- 

 rudite of Brogger from Grorud, only that the groundmass has a 

 greater degree of granularity. The occurrence of these quartz- 

 tinguaites in the Bearpaw mountains we have already indicated, 

 and one of them has been described.* The rock here men- 

 tioned is closely similar to the one previously described, only 

 that in the former specimen the feldspars are uniformly larger 

 and more tabular in their habit. 



Pseudo-leucite-sodalite-tinguaite. — About a half mile beyond 

 the southern boundary of the Beaver Creek stock a dike of 

 peculiar bright green rock was observed cutting the altered 



* This Jour., vol. 1, p. 394, 1895. 



