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Weed and Pirsson — Highwood Mts. Laccoliths. 



pj Xfral/anl late. _ .,,- 



are the deep erosion of the margins of the valleys into bad- 

 lands, of those of the igneous masses into fantastic rock piles, 

 and the many heavy, black dike walls which cross the open 

 stretches or form buttresses along the slopes. 



A petrographical description of the sodalite-syenite of 

 Square Butte by Lindgren and Melville* is the first publica- 

 tion relating to the geology of this part of the Highwood Mts. 

 area which has come to our attention. Its geology and 

 petrography have been fully described by the writers in a later 

 paper,f and the geology of the area as mapped by the authors 

 has been published in the Fort Benton Folio of the Geologic 

 Atlas of the United States accompanied by text briefly describ- 

 ing the geology by Mr. Weed. 



Geologically the area differs from that of the rest of the 

 Highwoods in that it is one of intrusion devoid of extrusive 

 igneous masses. The evidence at hand does not show 



that here the magmas ever 

 attained the surface and pro- 

 duced volcanic outbreaks ; 

 if such occurred, the extru- 

 sive material has long since 

 been swept away by erosion 

 and the facts point rather 

 to the view that this has 

 not occurred. 



The igneous masses are 

 divided into the Square 

 Butte, Palisade Butte, and 

 the Shonkin Sag laccoliths, 

 and the dikes and intrusive 

 sheets, as seen on the map 

 figure 1. 



Although topographically 

 and in size it is the least of 

 the laccoliths and, at first 

 sight, of the least impor- 

 tance, we shall commence with an account of the Shonkin Sag 

 laccolith for the reason that geologically it is the most impor- 

 tant and supplies the key to certain features which they all 

 possess in common. Moreover, it is in several features and 

 in the character of its dissection by erosion one of the most 

 perfect laccoliths that has been described, and its internal 

 structure is such as to render it an occurrence of great 

 importance in theoretic petrology. 



*This Journal, vol. xlv, 1893, p. 289. 



\ Bull. Geol. Soc. of America, vol. vi, 1895, pp. 389-422. 



3 



*¥" 



Syenite. Shonkinite. Cretaceous. Dikes 



and sheets. 

 Fia. 1. Map of Laccoliths. 



