1896.J Petrography. 299 



rocks form a natural series with sanidinite and peridotites. Rocks 

 composed of orthoclase and no augite = sanidinite ; when orthoclase 

 exceeds augite = augite-syenite ; when orthoclase equals augite = 

 yogoite ; when augite exceeds orthoclase = shonkinite ; when augite 

 alone is present = pyroxenite and peridotite. In this scheme the term 

 augite includes also other ferro-magnesian minerals, and the terms 

 orthoclase other feldspars. 



In connection with the article above referred to Iddings* mentions 

 the existence of a series of rocks associated with typical basalts and 

 andesites in the Yellowstone National Park. They represent like 

 phases of differentiation belonging to separate, but similar rock fami- 

 lies. Most all of these rocks are basaltic looking. They occur in flows 

 and dykes and sometimes as breccias, constituting the major portion of 

 the Absaroka Range. These rocks present a wide range of 

 within definite limits, forming a series connected by gradual transitions. 

 Three classes are distinguished, the first of which is characterized 

 usually by abundant phenocrysts of olivine and augite and an absence 

 of feldspar phenocrysts ; the second class is characterized by the pres- 

 ence of labradorite phenocrysts in addition to those of olivine and 

 augite, and the third class by the presence of labradorite phenocrysts. 

 The names given to the three classes are absarokite, shoshonite and 

 banakite. The distinctions between the classes is based principally 

 upon their chemical relationships. A large number of analyses, most 

 of which were taken from other papers, illustrate their points of differ- 

 ence. A comparison of the analyses, besides showing the close relation- 

 ships existing between the rocks of the three classes, shows also what 

 mineralogical differences may obtain for rocks of the same chemical 

 composition. The shoshonite from the base of Bison Peak and the 

 banakite from Ishawooa Canyon have practically the same chemical 

 composition. The former, however, contains abundant phenocrysts of 

 labradorite, augite and olivine, while the latter contains numerous 

 labradorite phenocrysts, but few and small ones of the other two min- 

 erals. The groundmass of the first shows much less orthoclase than 

 that of the second, and no biotite, which abounds in the second. The 

 author compares the series of rocks studied by him with the series 

 studied by Merrill 5 , with the series discussed by Weed and Pirsson and 

 with Brogger's 6 giorudite-tinguaite series. The conclusion reached by 

 this comparison is to the effect that it maybe doubted whether the gen- 



* Journal of Geology, Vol. Ill, p. 935 



«Cf. American Naturalist, 1895, p. 567. 



