GQ GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



the microstructure and grain of the rock from the summit and south slope 

 of Mount Holmes (77, 78), and those of the rock from the "White Peaks west 

 of the head of Indian Creek (80, 81, 82), are very uniform, and represent the 

 coarsest-grained forms found. The highest part of the mass as it now 

 exists, the summit of Mount Holmes, is nearly as coarsely crystallized as 

 any part of the body examined. 



The coarsest varieties consist of quartz in allotriomorphic individuals, 

 inclosing nearly idiomorphic crystals of feldspar, with fewer of biotite and 

 magnetite. The structure is thus micropoikilitic, or is almost exactly 

 analogous in the relative proportions and sizes of the crystals to microphitic 

 structure in ophitic basalt. The quartz extinguishes light between crossed 

 nicols throughout small areas, in which are scattered more or less rectangular 

 and lath-shaped feldspai-s. The quartzes are colorless and have few inclo- 

 sures of liquid with moving bubbles and salt cubes. The feldspars are 

 cloudy and partly altered ; hence the minerals are easily distinguished. The 

 structure is shown in PI. XI, fig. 3. The feldspars are partly unstriated. in 

 Carlsbad twins, with low extinction angles and low double refraction; these 

 are probably orthoclase; others are partly striated, in polysynthetic twins, 

 with low extinction angles, and are lime-soda feldspars, probably oligoclase. 

 It appears as though the latter predominated. The few phenocrysts are 

 striated lime-soda feldspars. Owing to the low percentage of calcium oxide 

 in the rock, the feldspars must correspond to plagioclase rich in soda. In 

 one instance the feldspar contains inclusions of what appears to be glass, 

 but its exact nature is doubtful. Biotite occurs in six-sided plates and as 

 irregular individuals, with brown color and strong absorption. They often 

 contain minute magnetite grains. The biotite is sometimes in small aggre- 

 gates with magnetite, which also occurs in scattered crystals. Apatite is 

 present in long slender prisms, but is rare. In some cases it exhibits a 

 distinct blue and brownish-purple pleochroism. Minute zircon prisms are 

 present. 



Among the secondary minerals is a little muscovite in radiating tufts. 

 Chlorite, resulting from the alteration of biotite, is occasionally noticed. 

 The decomposition of the feldspar produces a dust-like, indeterminable 

 mineral, white by incident light and yellowish in transmitted light. 



In the dacite-porphyry of Echo Peak there are occasional inclosures 

 of what seems to be coarse-grained gneiss. The only specimen examined, 

 however, shows a coarse-grained rock with somewhat gneissic structure, 



