80 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK. 



end of a large hornblende is normally developed where it lies within the 

 groundmass of the rock, but Avithin the banded segregation it is poikilitic, 

 and is traversed by a broad band of feldspar grains. In places where the 

 banding of the magnetite and biotite is most pronounced and is in delicate 

 lines, the feldspars are comparatively large and cross the lines of magnetite 

 at all angles without modifying them. The lamination was earlier than 

 the crystallization of the feldspars. It did not affect the orientation of the 

 hornblende material. It must have preceded the crystallization of the 

 hornblende. It affected the size of the feldspar crystals in part. The size 

 of crystals depends primarily upon the rate of cooling or upon the viscosity 

 of the magma. Since the cooling must have been the same for all parts of 

 so small a mass, the most variable factor is likely to have been the viscosity, 

 which depends not only on the temperature but on the chemical composi- 

 tion. Hence we may conclude that heterogeneity in the mass must have 

 been the cause of the banding and abnormal microstructure in these small 

 gneiss-like segregations. 



HORNBLENDE-PYROXENE-ANDESITE-PORPHYRIES AND ANDESITES. 



The hornblende-pyroxene-andesite-porphyries and andesites are few in 

 number and are closely associated with the hornblende rocks. One forms 

 a narrow sheet on the ridge north of the head of Fan Creek (162, 167). It 

 is dense and andesitic in appearance, with hornblende phenocrysts. The 

 groundmass is holocrystalline and pilotaxitic, with abundant magnetite. 

 Part of the rock has small phenocrysts of augite in addition to those of 

 hornblende, besides numerous small augites, first recognized under the 

 microscope. The groundmass is more coarsely crystallized. 



Two other modifications occur, whose character is in doubt, owing to 

 the complete decomposition of the ferromagnesian constituents. They were 

 probably pyroxene rocks (131, 132, 134, 136). They are both fine grained, 

 without phenocrysts. Their general mineral composition is similar to 

 that of the rocks alreadj^ described. Their microstructure is andesitic, 

 approaching a microlitic and glassy structure. Some of the pseudomorphs 

 have the shape of olivine, and it is possible that one of these rocks is an 

 altered basalt or olivine-andesite. 



