CHAPTER IX. 



ABSAROKITE-SHOSHONITE-BANAKITE SERIES. 



By Joseph Paxson Iddings. 



INTRODUCTION. 



There are certain basaltic-looking rocks associated with the older 

 andesitic breccias in various localities within the Park, which form lava 

 flows in most cases and dikes in others, and which more rarely constitute 

 part of the basic breccia itself. They are usually quite subordinate in 

 amount, judging from the extent of their exposures, but, considering' the 

 probable size of their original masses, before being exposed by erosion 

 they must have had very considerable volumes. As flows they are basaltic 

 in character, being dark colored and heavy, with olivine among the pheno- 

 crysts in most cases. They are massive and compact or vesicular. They 

 are porphyritic in some cases, but not noticeably so in others, and generally 

 exhibit a semiwaxy luster that suggests the presence of nepheline, which, 

 however, is not present. The waxy luster is due to the alkali feldspar in 

 the groundmass. They are often dull greenish black owing to the serpen- 

 tinization of olivine. As dikes they are basaltic in some cases and trachytic 

 in others, being gray in various shades, and having a somewhat waxy luster 

 in the rocks of darker shades. The luster, as in the case of the flows, is 

 due to the alkali feldspars, as there is no nepheline. They are porphyritic 

 or not in different cases, and range from aphanitic to phanerocrystalline. 

 As will be seen later, they represent a rather wide range of composition, 

 both chemical and mineralogical, and though ■ genetically related and con- 

 nected by gradual transitions, so that they constitute a natural group, they 

 could not be embraced by any one definition and must be divided into 

 several classes. 



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