398 WEED AND PIRSSON — HIGHWOOD MOUNTAINS OF MONTANA. 



platy fragments, forming a talus slide, but rarely ma,kes bold craggy 

 masses rising abruptly from the slopes. Nowhere was the main body of 

 granular rock seen cut by dikes. 



An interesting feature of the Shonkin core occurs at its extreme south- 

 ern end. Here there is what is believed to be a volcanic throat. The 

 coarse grained rock occurs in great blocks, often with partially rounded 

 edges, forming a coarse agglomerate whose matrix is a finer grained 

 variety. The tumultuous arrangement of the agglomerate, the coarsely 

 granular nature of the rocks, and the baking of the adjacent basaltic 

 breccias all indicate a volcanic throat. 



It is the only locality in the Highwoods showing evidence of solfataric 

 decomposition, the rocks adjacent to the neck being altered and iron- 

 stained. This neck represents probably one of the latest points of vol- 

 canic activity of the mountains, as it breaks through the basic breccias. 



Arnoux Core. — This is a comparatively small core of granular rock 

 breaking through acidic tuffs and basaltic lavas. There are a number of 

 dikes traceable to this center, and it is surrounded by a ring of meta- 

 morphosed rocks. The core material resembles that of the Shonkin 

 center, which is near by, and from which it does not differ in any essential 

 characteristic. 



Palisade Butte. — The eastward extension of the range is formed by two 

 isolated points. Square and Palisade buttes. The latter and smaller of 

 these stands about midway between the group of breccia hills which 

 forms the eastern foothills of the main mountain mass and Square butte. 

 It consists of a huge monolith of rock rising some 600 feet above the 

 slopes at its foot, the whole mass having an elevation of about 1,000 feet 

 above the plain on which it stands. The breadth across the isolated rock 

 mass is about three-quarters of a mile and the shape is nearly circular. 

 On the top it is not horizontal, but slopes dow^nward from south to north 

 at an angle. The rock mass rises abruptly above its pediment and fronts 

 the plain on all sides in a series of bold precipices. These precipices are 

 not, however, continuous either around the circumference or from top to 

 bottom, being broken by a series of narrow, steep ravines which permit 

 access to the summit. 



The most striking feature of the butte is the magnificent columnar 

 structure of the igneous rock composing it and from which it has received 

 its name. The steep precipices composing the front are formed of a vast 

 series of huge regular perpendicular columns, each of which rises regu- 

 larly from the talus and continues to the top. These columns are some 

 18 inches in diameter and may be 150 to 200 feet high. They evidently 

 once extended the whole height of the mass. The butte thus bears a 

 striking resemblance to the well known Devils Tower in the northwest- 

 ern part of the Black Hills region. 



