290 Weed and Pirsson — Bearpaw Mountains, Montana. 



ble that it may be original and represents extremely fine 

 inclusions, which, had they been gathered into larger grains, 

 might have arranged themselves zonally. We believe, however, 

 that they really represent a stage of alteration and that the 

 leucites, since they appear so exactly like kaolinized orthoclase, 

 are in fact partially kaolinized. That some alteration has 

 taken place seems possible from the fact that the rock contains 

 over 3 "5 per cent, of water, though this in part may well come 

 from the including base. Qualitative tests showed the absence 

 of SO, and CI, and the mineral is therefore not hauyn, nosean, 

 nor sodalite. 



The rock appears closely related to the analcite basalts of 

 the High wood Mountains which have been described by 

 Lindgren,* who showed that in the material studied by him 

 the mineral resembling leucite was really analcite, and from 

 the freshness of the other components he was inclined to regard 

 it as of primary origin. In the course of our own work in the 

 Highwood Mountains we have gathered a large and varied 

 collection of these leucite-like rocks and have found them to 

 consist of several types, and we hope that the investigation 

 which we are now making of them will throw some light on 

 the character and origin of the leucite-analcite minerals. The 

 discussion of these similar Bearpaw types is deferred in conse- 

 quence until our publication upon this material, which will ap- 

 pear at a later date. 



The ground-mass in which the other minerals lie is very 

 small in amount and consists mainly of minute black grains of 

 magnetite, with tiny microlites of augite imbedded in what 

 appears to be a colorless glass. 



The composition of the rock from the foregoing is thus 

 shown to be a very simple one — magnetite, augite, olivine, and 

 probably altered leucite. There are no signs of any feldspar 

 .present, and the rock is thus a leucite basalt. The occurrence 

 of leucite in this locality we shall have occasion to mention 

 later, in connection with its presence in some remarkably fresh, 

 unaltered, and interesting lavas from Bearpaw Peak. 



Intrusive Hocks. 



Eagle Butte. — The eastern end of the mountains, near 

 Cleveland, is formed of the sedimentary rocks tilted near the 

 postoffice at angles of 15° to 20° to the southeast, while the 

 prominent buttes seen near by are formed of igneous intrusions 

 of basaltic rocks. The extreme easterly point of the moun- 

 tains is an isolated conical hill of considerable elevation, called 

 Eagle Butte on the map. It shows a cap of black debris 



* 10th Census U. S., vol. xv, p. 719. Proc. Cal. Acad. Sci., Ser. 2, vol. iii., p. 51. 



