TH E 



AMERICAN JOURNAL OF SCIENCE 



[FOURTH SERIES.] 



Art. XLYI. — Missourite, a new Leucite Rock from the High- 

 wood Mountains of Montana', by Walter H. Weed 

 and Louis Y. Pirsson.* 



The Higliwoods form one of the isolated mountain groups 

 of central Montana which rise like islands from the great tree- 

 less plains stretching eastward from the slopes of the Rocky 

 Mountain Cordillera, and forming the great basin of the 

 Missouri River. They consist of a group of extinct, greatly 

 eroded volcanoes, and the elevations which now compose the 

 area are formed chiefly of tuffs, breccias, and lava flows resting 

 on Cretaceous sediments, together with intruded stocks or cores 

 of massive granular rocks which represent the former centers 

 of volcanic activity and from which great numbers of dikes 

 radiate outward in all directions.f 



In the preparation of a report on the geology of this moun- 

 tain group it has been found that the body of granular rock 

 forming the core at one of the denuded volcanic centers is com- 

 posed of a new rock type whose petrologic character is of 

 exceptional interest. As the type, moreover, proves to be of 

 great importance to systematic petrography, it has been thought 

 best to present a brief account of the rock and its mode of 

 occurrence, a more detailed description and the discussion of its 

 geological and petrographical relations being reserved for the 

 report in preparation. 



* By authority of the Director of the U S. Geological Survey. Field geology 

 and collection of material by W. H. "W. ; microscopical petrography by L. V". P. 



f A sketch of the geological features of the region, with a geological map, has 

 already been pubUshed by the authors. Bull. G-eol. Soc. America, vol. vi, p. 

 389, 1895. 



Am. Jour. Sci.— Fourth Series, Vol. II, No. 11. — November, 1896. 

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