130 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



The hornblende is sometimes represented by paramorphs of magnetite and 

 augite in the pyroxene-andesites, often exhibits black borders in the inter- 

 mediate andesites, and in the more acid andesites and dacites is entirely 

 free from any dark border. Its form and colors are the same as before 

 described for the andesites of the breccias. Biotite is also the same as in 

 those rocks. In one instance it incloses a small crystal of plagioclase. 



The feldspars all exhibit polysynthetic twinning. Their cross sections 

 are mostly rectangular in the more basic andesites, and are labradorite. In 

 some of these andesites they appear to be oligoclase. In the more acid 

 andesites and dacites the lime-soda feldspars are larger and have more 

 crystal faces. They appear to belong to several species. Besides numerous 

 glass inclusions, there are a few instances in which feldspar phenocrysts 

 contain opaque needles and grains, arranged in several systems of parallel 

 lines, which are identical with the inclusions in many of the labradorites in 

 the diorite of Electric Peak. They are sometimes accompanied by glass 

 inclusions, which proves their primary character. Quartz phenocrysts are 

 both idiomorphic and rounded in the same rock section. They usually 

 occur in isolated grains, but sometimes several are attached to one another 

 (PI. XXI fig. 2), just as several feldspars often are. Glass inclusions are 

 abundant. In only one case were fluid inclusions noticed together with 

 those of glass. Magnetite, apatite, and zircon occur in their usual forms 

 and in ordinary amounts. 



The groundmass of these rocks differs in degree of crystallization, in 

 mineral composition, and in structure. In the more basic andesites it is in 

 many cases glassy, with multitudes of microlites of pyroxene, plagioclase, 

 and magnetite; in many others it is completely crystallized and the out- 

 lines of the microlites are no longer sharply defined. In the holocrystalline 

 varieties of these rocks the different degrees of crystallization may be 

 compared with one another by arranging them in a table according to the 

 size of grain of the groundmass. This has been done in Table XIII, in 

 which they have been combined with the specimens of breccia from Sepul- 

 chre Mountain, and have been separated into mineralogical groups whose 

 scope may. be seen by comparison with Tables XI and XII. The grades 

 of crystallization correspond to those established for the intrusive rocks of 

 Electric Peak, with the addition of five more grades, which embrace two 

 finer-grained degrees of holocrystalline structure and tlu-ee degrees of glassi- 



