THE 



AMERICAN JOURNAL OF SCIENCE 



[FOURTH SERIES.] 



Art. I. — Geology of the Shonlcin Sag and Palisade Butte 

 Laccoliths in the Highwood Mountains of Montana / by 

 W. H. Weed and L. V. Pibsson.* 



Introductory. — On the southeast side of the Highwood 

 Mountains, the open treeless plains country which stretches 

 away in all directions from the foot of the broken volcanic 

 hills of tuffs and lava, has its monotony interrupted by a tract 

 of broken and sharply accidented character. Although sepa- 

 rated from the rest of the mountain group by certain geolog- 

 ical features not elsewhere seen and by its outlying position, it 

 is nevertheless an integral portion of the Highwood eruptive 

 area since it belongs to the same geologic period of eruptive 

 activity and is merely a prolongation of the eruptive masses in 

 this direction, separated, like the rest of the mountains, from 

 the other igneous groups of central Montana by wide stretches 

 of sedimentary rocks which are undisturbed by such intru- 

 sions. 



This area is terminated and its accidented character accentu- 

 ated by the deep trench-like valley which Arrow River has 

 scored in the plain and where, in the deeply carved band of 

 badlands which decorates its margin, are revealed the parti- 

 colored strata of the level-lying Cretaceous. 



Topographically and scenically, this region is characterized 

 by the dark frowning masses of Square and Palisade Buttes, 

 which form such prominent landmarks across the level horizon 

 and weary miles of the vast open prairies, and by the debouche- 

 ment of the shallow valley of the Shonkin Sag, elsewhere 

 described, into that of the Arrow River ; while minor features 



* By permission of the Director of the U. S. Geological Survey. 



Am. Jour. Sci.— Fourth Series, Vol. XII, No. 67.— July, 1901. 

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