I 



Weed and Firsson — Bearjpaw Mountains^ Montana. 143 



related to the nephelite basalts, it has been referred to that 

 group. 



So far as our knowledge goes, these rocks have been 

 described from only two occurrences in this country. The 

 iirst of these is at Pilot Knob in Texas, described by Hill 

 and Kemp,^ and the second is in the Cripple Creek district in 

 Colorado, where their presence in dikes has been mentioned 

 by Cross.f Allied rocks have been described by Zirkel :j: from 

 the Kawsoh and Elkhead mountains, but since they contain 

 plagioclase it would appear that they should rather be referred 

 to the tephrites. 



Leucitite of Bearpaw Peak. 



The highest summit of the mountains is known as Bearpaw 

 Peak. It is situated in the western part of the region, and its 

 dark conical point, rising far above the neighboring summits, 

 is the most prominent feature of the mountains. This prom- 

 inence is due to its greater height (7040 feet) and to the fact 

 that it stands apart from the adjacent summits, from which it 

 is separated by meadows and a lower hilly region in which the 

 numerous streams radiating from the peak find their sources. 

 From the top there is an excellent outlook over the entire 

 mountain area. The smooth grassy slopes and lack of rocky 

 exposures seen everywhere convince one that rapid geological 

 mapping is not possible ; each point must be actually visited 

 to ascertain its nature, l^o actual dikes are seen, but the 

 grassy lines seaming the butte immediately west of the peak 

 represent minette-like dikes cutting metamorphosed sedimen- 

 tary rocks. 



The mountain is formed of extrusive rocks, which are all 

 basaltic and consist of varying forms of scoria, breccia, and 

 bedded tuffs with occasional intercalated lava flows. Good 

 exposures are not rare, though the summit and sides of the 

 mountain are covered by a mantle of debris extending far 

 down its slopes. 



The rocks are leucite basalts, varying as greatly in color as 

 in physical nature and presenting different phases of this type 

 of rocks. The prevailing type is similar to that described in 

 the first part of our former paper. 



About midway upon the northern flanks of the mountain a 

 dark-gray, almost black I'ock occurs. A prospect is located 

 near by, though there was no sign of ore seen. The rock 



* Am. Geoloo:ist, Nov. 1890, p. 286. 



I Pike's Peak Folio. Geo!. Atlas, No. T. U. S. Geol Survey, 189i. Also Geol. 

 Cripple Creek Dist , 16th Ann. Rep. U. S. Geol. Surv. 1895, p. 49. 

 X Surv. 40th Parallel, vol. vi, Micro. Petrog., pp. 255, 256, 1876. 



