316 Weed and Pirsson — Missourite^ a neio Leiicite 



The stock or core is situated at the head of Shonkin Creek, 

 a large stream draining the northern part of the mountains. 

 The headwaters of this stream have cut deeply trenched chan- 

 nels through the mountains and have exposed the granular rock. 

 The region, although mountainous, is almost devoid of timber. 

 Smooth grassy slopes with occasional low rock exposures gener- 

 ally prevail. 



Geological ocGurrence. — The new rock type described forms 

 a stock of granular rock intrusive in Cretaceous shales and in 

 the fragmental volcanic material which overlies them, both being 

 highly altered near the contact with the igneous mass. These 

 enclosing rocks are cut by a multitude of dikes, radiating from 

 the core as a center and forming the most conspicuous feature 

 of the surrounding country. 



The igneous rock forming the stock constitutes an irregular 

 mass 2^ miles long and in places half as wide. Where covered 

 by the sedimentary strata, the structure simulates that of a 

 laccolith, but careful study showed that the intrusion is not of 

 this character. The igneous rock was in part intruded between 

 the sedimentary rocks and the volcanic breccias which overlaid 

 them, and in part injected along the bedding planes of the sedi- 

 mentary strata at the edges of the stock. At the south end of 

 the core a coarse agglomerate of massive rock represents the 

 filling of a vent of a volcanic throat, the material of the blocks 

 and cement varying greatly in granularity but consisting essen- 

 tially of the same type composing the main body of the core. 



Constituting beyond all doubt a geological unit, the rock 

 mass of this volcanic stock varies considerably both in coarse- 

 ness of grain and in the proportion of its constituent minerals. 

 The specimen selected for description and analysis represented 

 the coarsest grained and freshest variety observed. 



The rock seldom forms conspicuous exposures ; near the 

 contact it is sometimes weathered into castellated masses and 

 pinnacles, but the usual outcrop is low and hidden by the debris 

 blocks into which the rock ordinarily weathers. Platy part- 

 ing was observed near the contact, but elsewhere the fracture 

 is massive and determined by shrinkage planes. 



Megascopic characters. — Seen in the outcrop, the rock appears 

 dark gray, coarse grained, and resembles many basic massive 

 rocks in appearance. In the specimen it is seen to be coarsely 

 and evenly granular and to be composed of light and dark con- 

 stituents, the proportion by bulk being about two of the light 

 to three of the dark minerals. The separation by the heavy 

 fluids shows, however, that by weight the white mineral forms 

 only one-fifth to one quarter of the whole. The distinction in 

 color is strongly marked and gives the rock a mottled mosaic- 

 like appearance. 



