Weed and Pirsson — JBearpaw Mountains, Montana. 301 



dense ground-mass when treated with acid is found to yield a 

 small quantity of gelatinous silica, and as no other mineral 

 appears to be present which could yield it, it is probable that 

 this comes from a small quantity of nephelite not otherwise to 

 be detected. 



This rock belongs then very plainly in the alkali syenite 

 series, and is a porphyry with a somewhat trachytic habit. 

 The contrast between the size of the feldspar phenocrysts in 

 the main type and in the contact form indicates that they were 

 formed in place and not brought up from below, which idea is 

 favored by the vastly greater number of them in the contact 

 type. This is analogous to the facts shown at Gray Butte and 

 mentioned elsewhere in this paper. 



The shaft of the mine has been sunk upon a mass of shonkin- 

 ite, a dark, basic, micaceous rock which appears to form a 

 dike cutting transversely across both the periphery of the 

 trachyte intrusion and its marginal zone of hardened, baked 

 sedimentary rocks. This rock is of a moderately coarse grain, 

 and is filled with stringers and thin seams of pyrite. It con- 

 sists of iron ore, apatite, augite, biotite, and soda orthoclase 

 feldspar. It is moderately coarse-granular and the augite and 

 biotite which are the chief ferro-magnesian minerals are 

 enclosed poikilitically by the alkali feldspar in broad plates. 

 The structure is hypidiomorphic and the rock is identical with 

 a similar one from an intrusive center on Beaver Creek, and to 

 avoid repetition further details are deferred to the more com- 

 plete petrographic description of the type occurring at the 

 latter locality. 



Its association, however, with the syenite is a matter of great 

 interest, and shows that it is here the complementary rock 

 to that type. The complementary relation between syenite 

 and shonkinite we have already shown on two occasions,* and 

 the present example will be included in the second part of 

 this paper, where we will present further cases in the Bearpaw 

 Mountains with more complete description and a fuller dis- 

 cussion. 



The gangue of the ore body is a brecciated and much altered 

 trachyte or syenite-porphyry. The fragments composing it 

 are angular, of varying size, color, and character, and the rock 

 shows considerable pyrite scattered throughout its mass. 

 Examined under the microscope, the thin section shows 

 un twinned feldspar phenocrysts, biotite, and iron ore in a 

 feldspathic ground-mass consisting apparently of singly or 

 untwinned alkali feldspars. It is now so greatly altered by 

 leaching solutions, filled with calcite which exists everywhere 

 in thin films, and the feldspar is so changed into sericite, that 

 it would not be safe to assert more than this about it. 



* Bull. Geol. Soc. Am., vol.vi, p. 400, 1895; this Jour., vol. 50, p. 46T, 1895. 



