240 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK. 



in the earlier history of the volcano this variation was within such a part of 

 the chemical scale of variation that the resulting rocks might be called 

 andesites or basalts, but that this difference of name corresponds to no 

 greater difference in composition than that between the varieties of basalt 

 of the later period. It is also probable that flows of pyroxene-andesite 

 occur among the lava streams, but that they were not distinguished in the 

 field. 



The greater part of the basalt flows are andesitic in habit — that is, in 

 microstructure — and are like the basalts forming the breccias. None of them 

 are ophitic. The groundmass is in most cases glassy; in others, holocrystal- 

 line. Some of them carry large phenocrysts of feldspar and resemble the 

 same variety of breccia in microstructure. Some contain a little orthoclase in 

 the groundmass, and are intermediate between normal basalt and shoshonite. 



INTRUSIVE ROCKS. 

 OUTLYING DIKES. 



As already pointed out, the dikes belong to several converging- groups, 

 the largest of which centers in the gabbro core and a smaller one in a focus a 

 few miles east, while a great number of dikes in the southern part of the 

 district belong to an outlying volcanic center situated near the headwaters 

 of Stinkingwater River. 



The rocks constituting these dikes exhibit more variation than the brec- 

 cias, though the majority of them are like the breccias in composition and 

 habit, being basalt, But toward the end of the period of volcanic activity, 

 as we learn from the structure of the granular core, the composition of the 

 magma became more and more siliceous, and the volume of the lava 

 erupted, or the size of the fissures from which we estimate this volume, 

 became smaller. At the same time we learn from certain dikes that peculiar 

 phases of the magma made their appearance, the rocks of which deserve 

 special consideration. It is to be remarked that while the most siliceous 

 modifications of the magma occur within the core, the most basic phases of 

 it are found at considerable distances from the center, with one exception. 

 This accords with the idea that the more siliceous products of differentiation 

 will occur near the center of the reservoir in which differentiation takes place, 

 presumably beneath the crater of a volcano, while the less siliceous products 



