76 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK. 



feldspars with low extinction angles. Some exhibit no twinning. It is 

 probable that oligoelase and orthoclase are both present. Irregularly shaped 

 grains also occur. Quartz is present as poikilitic cement, or in minute grains 

 when this structure is not developed. Magnetite crystals and some irregular 

 shreds of biotite and green hornblende also take part in the groundmass. 

 In this mass lie small crystals of feldspar and the ferromagnesian minerals, 

 varying in size to the largest phenocrysts. In one specimen small quartz 

 phenocrysts occur (147). The groundmass of the rock forming the western 

 summit of Electric Peak is micropoikilitic, with more hornblende and biotite 

 as constituents (197). The same structure is found in an altered sheet on 

 the northeast spur of this mountain (214). Slightly finer-grained forms of 

 this same structure occur in two other sheets in Electric Peak (201, 151). 

 A lower grade of crystallization has smaller and less distinctly poikilitic 

 grains, with clearly defined lath-shaped feldspars that sometimes exhibit a 

 fluidal arrangement. A still lower grade consists of minute lath-shaped 

 feldspars, with indistinct patches of poikilitic quartz, and spots that are 

 microcryptocrystalline. There is sometimes pronounced flow structure. 

 In thin sections the groundmass is gray, with minute feldspars, and mag- 

 netite grains, and occasional apatites. These modifications are but slightly 

 different from andesites, and might be classed as such. They occur in 

 varieties richer in biotite than in hornblende. A microstructure very similar 

 to this, but less uniformly developed and still finer grained, is found in a 

 crushed sheet in the southeast spur of Electric Peak (213), where the rock 

 is considerably altered. In this case it is possible that the micropoikilitic 

 structure is secondary. The lowest grade of crystallization allied to the 

 micropoikilitic is one in which there is a brownish base, which is doubtfully 

 holocrystalline. In this are feldspar microlites and irregular grains, in 

 places approaching a poikilitic structure. It is probable that the rock is 

 holocrystalline. Its habit is thoroughly andesitic. 



The microstructure of other modifications of these intrusive sheets is not 

 very different from the micropoikilitic varieties, in that it consists of lath- 

 shaped feldspars and grains of feldspar and quartz of nearly the same size 

 as in the other cases, but the poikilitic cementing quartz is wanting. These 

 rocks are very fine grained, the average diameter of the grains being about 

 0.02mm., or smaller. In one instance the granular groundmass is filled with 

 phenocrysts of feldspars and some micrographic intergrowth surrounding 



