396 WEED AND PIRSSON — HIGH WOOD MOUNTAINS OP MONTANA. 



considerable differentiation of magraatic material has taken place at each 

 of them and the granular rocks now exposed present several types of 

 interest. 



South and Highwood Cores. — On the western side of Highwood gap there 

 are two of these cores, and the most southern of them is the oldest center 

 in the range. It is composed of an augite-syenite, and rises through 

 level-bedded sandstones, which it has greatl}^ indurated. Large numbers 

 of dikes depart from its circumference radially in all directions. They 

 are generally rather narrow and basaltic in character and of two prevail- 

 ing types, one a possible leucite or analcite-basalt, with phenocrysts of 

 leucite, (?) augite and olivine ; the other type contains only very large 

 plates of black biotite in a dense black groundmass. The " comple- 

 mentary " dike-rocks, to use the excellent term of Brogger, are porphy- 

 ries, with large, thin, tabular phenocrysts of feldspar, which are much 

 less common than the basaltic types. 



This south core of massive rock is not surrounded by breccias or flows, 

 those to which it possibly gave rise having been carried away by an 

 amount of erosion sufficient not only to cut down the former cone en- 



COMB flUT7£ iOuTH COKE HICHWOOO aAP LAVA PS/IK tAiT COUE 



]^^^iMsyihinE !•:■:'$■■: '■■I tuffs and flows ^^S cf!BT»c sous beds 



Figure 2.— Cross-section through Highwood Mountains. 



Vertical and horizontal scales the same. 



tirely, if such ever existed, but also to erode deeply into the level strata 

 through which the former vent arose. This denudation left the strata as 

 a small range, through which the later outbursts forced their way and 

 whose slopes the}^ covered with extrusive material. These outbreaks 

 took place at Highwood peak, whose exposed core, denuded of its cover- 

 ing of ashes, tuffs, etcetera, now towers up as the highest and sharpest 

 peak in the group. 



During the period of activity of this volcano it emitted at first acidic, 

 light colored feldspathic tuffs and breccias, intermingled w^ith flows of 

 felsite and possibly phonolite. These in turn were succeeded by ejec- 

 tions of very basic matter, which formed breccias and lavas of basalt 

 similar to the South Peak dikes. 



The Highwood core may be also said to be a syenite, but it presents 

 a number of points of great petrologic interest, giving evidence of a re- 

 markable amount of differentiation in place. It also is surrounded by a 

 number of radial dikes similar in character to those mentioned before. 



East Core. — The east core, at the head of Davis creek, appears to be 

 still younger. It is still partly surrounded by the masses of basaltic 



I 



