140 Weed and Pirsson — Bearpaw Mountains^ Montmia, 



in the same crystal, the green surrounding a colorless core ; at 

 other times the colors are irregularly mingled. The largest 

 crystals of pyroxene appear to be partly resorbed ; these are 

 rather rare, while a smaller generation is abundant in the 

 groundmass. Small prisms of the pyroxene are common inclu- 

 sions in the large feldspar phenocrysts. 



The hiotite appears also in two generations ; the large crystals 

 which attain l"^"" in diameter are rare ; they are irregular and 

 embayed and appear as if partly resorbed. It is strongly ple- 

 ochroic between a deep chocolate-brown and colorless. A 

 slight opening of the axial cross in a very thin basal section 

 permits us to determine it as meroxene. In the second gener- 

 ation it is freely scattered through the groundmass in small 

 shreds and patches. The iron ore, titanite, and apatite which 

 occasionally occur offer no peculiarities worthy of notice. 



The structure of the groundmass is allotriomorphic, granu- 

 lar, and from what has been said it is clear that the rock 

 might be classed as a syenite porphyry which has in the hand 

 specimen a trachytic appearance and habit. Since, as has been 

 previously mentioned, it is certain that the Bearpaw rocks are 

 of Tertiary age, it follows that by those who still recognize 

 the age distinction in petrography, it would indeed be classed 

 as a trachyte. We have, however, in these papers used the 

 terms in a purely structural sense, without reference to age. 



Nephelite Basalt Intrusion^ near Lloyd. 



A couple of miles south of Lloyd on the headwaters of 

 Snake Creek, a dark-colored rock is exposed by the roadside 

 and is seen outcropping on the slopes to the east. The exact 

 geological relations were not determined, but it is intrusive in 

 slightly tilted, soft Cretaceous shales, which are somewhat 

 metamorphosed near the contact. The rock occurs in quite 

 large blocks, breaking with conchoidal fracture. It was recog- 

 nized in the field as being of unusual character, and this is 

 confirmed by petrographic study. 



On a fresh fracture it is a very dark stone-gray color and is 

 quite thickly dotted with small, well crystallized prisms of 

 black augite, which vary from 1 to 2'''°^ in length and are of 

 stout, thick habit. Much more rarely yellow grains of olivine 

 are seen of about the same size. The groundmass has a some- 

 what greasy appearance, whose dark-gray color gives the rock 

 its prevailing tint. It is very dense and compact, and under 

 the lens it can be seen to be of crystalline character. On 

 weathered surfaces the rock has a rusty brown color, dotted 

 and pitted by the altered augites. 



