MINERAL AND CHEMICAL VARIATIONS OF ROCKS. 259 



quartz with magnetite and altered pyroxene. The phenocrysts vary in 

 amount and size, and are plagioclase, augite, and hypersthene, with larger 

 crystals of hornblende, which is brown and brownish green and in one of 

 the rocks has a narrow border of magnetite. 



Lamprophyric rocks. — The remaining dikes do not constitute a distinct group 

 which may be sharply separated from the majority of the rocks of the dis- 

 trict, though certain of them possess marked characteristics. Their chief 

 distinction is an unusual mineral combination, but they are connected with 

 the ordinary rocks of the district by mineralogically intermediate varie- 

 ties. In general appearance they resemble the rocks Avith which they are 

 associated. 



They are fme-gTained rocks, characterized by an abundance of biotite 

 and other ferromagnesian minerals, including augite and olivine, with feld- 

 spar subordinate in some cases, and partly alkaline, while analcite appears 

 as a secondary mineral in some modifications of the rock. In other cases 

 the rocks are distinctly feldspathic. Because of their unusual composition, 

 and of the occurrence of similar rocks in dikes and lava flows in other parts 

 ' of the Yellowstone Park, their petrographical description is deferred to Chap- 

 ter IX, where they are classed as absarokites, shoshonites, and banakites, and 

 are considered as exceptional facies of the normal magma of the region. 

 They seem to represent less common differentiations of this magma than 

 the more numerous varieties of igneous rocks do, and for this reason may 

 be discussed separately. 



MINERAL AND CHEMICAL VARIATIONS OF THE ROCKS. 



The variations in mineral composition of the igneous rocks of this dis- 

 trict are evidently dependent on more than the chemical composition of the 

 magmas from which they have crystallized, rocks of the same or of similar 

 chemical composition having different mineral components, which has also 

 been shown to be the case at Electric Peak and Sepulchre Mountain. 



The range of mineral variation among the extrusive rocks, when con- 

 sidered in the order of their eruption, is from siliceous andesites with biotite 

 and hornblende, through those with hornblende and pyroxene, to pyroxene- 

 andesites, which grade into basalt by decreasing percentages of hypersthene 

 and increasing amounts of olivine. 



This is succeeded in a neighboring part of the country by a recurrence 

 of the same series from hornblende-mica-andesite to basalt, and finally by 



