DISTRIBUTION OF SHOSHONITE. 339 



olivine and augite, with fewer of labradorite. They are holo crystalline and 

 consist of plagioclase with orthoclase and some analcite, besides augite, 

 magnetite, and delicate needles of apatite. 



SHOSHOlSriTE. 



The rocks classed as shoshonites are more numerous than the absarokites, 

 and embrace a somewhat wider range of composition. They occupy the 

 middle ground, as it were, in this series, and pass by gradual transitions into 

 absarokite, banakite, and the normal basalts of the region. They occur 

 associated with absarokite, and consequently at the same localities. Their 

 principal occurrences are on the Lamar River, Mirror Plateau, in Crandall 

 Basin, in the Stinkingwater and Ishawooa canyons, on the southeast fork 

 of Beaverdam Creek, and at Two Ocean Pass, one of the heads of the 

 Shoshone or Snake River. This is the locality from which the rock was 

 first collected and identified as an orthoclase-basalt. The rock also occurs 

 on Grayling Creek and west of The Crags, southwest of Gallatin Mountains. 

 Most of the rocks are characterized by prominent phenocrysts of labrador- 

 ite, together with those of augite and olivine. A few are without feldspar 

 phenocrysts, but quite a number are without megascopic phenocrysts and 

 are correlated with the porphyritic forms on chemical as well as mineral- 

 ogical grounds. A small number are leucite bearing. The chemical com- 

 positions of eight of these rocks are given in the following table. To 

 these are added three analyses of transitional varieties. The analyses are 

 arranged so as to bring those most alike by the side of one another. As 

 already said, the mineralogical variations range from an abundance of 

 olivine and augite to a paucity of them, which corresponds to the chemical 

 variation from higher magnesia and lime to lower magnesia and lime, 

 and the corresponding changes in the alumina and alkalies in the opposite 

 direction. In describing these rocks those that are chemically and mineral- 

 ogically alike will be considered first, and afterwards those that are chemically 

 alike but mineralogically different. 



