DACITES OF ABSAROKA RANGE. 289 



The feldspar phenocrysts are all polysynthetic twins, and are lime- 

 soda feldspars, some of which exhibit symmetrical extinction angles indi- 

 cating labradorite, but others appear to belong to more alkaline species. 

 Zonal structure is highly developed. Inclusions are not abundant, and 

 consist of occasional crystals of biotite, hornblende, apatite, zircon, and 

 magnetite. The biotite, which is abundant, is in six-sided plates, and also 

 in relatively thick crystals, occasionally the thickness being greater than 

 the width. Its color is dark brown in thin cleavage plates, and strongly 

 pleochroic in sections perpendicular to the cleavage, being pale yellow for 

 light vibrating parallel to a, and almost opaque for that at right angles to 

 this direction. In places the lamellae of a crystal are bent at one end 

 where they approach another phenocryst, indicating the forcible crowding 

 of these crystals during the movement of the magma before solidification. 

 Inclusions of minute apatite crystals are common; those of magnetite and 

 zircon less so. The hornblende is less abundant than biotite, and is green, 

 with a tinge of brown, with marked pleochroism, pale brown || a, brownish 

 green || I), strong green || c. Crystal forms are seldom observed, except in 

 the prism zone, where the unit prism predominates, and both pinacoids 

 may be present. Often the outline is very irregular. Inclusions of biotite 

 occur, and sometimes apatite and magnetite. It is a later crystallization 

 than biotite. Apatite, magnetite, and zircon are present in their usual 

 forms. 



The groundmass is holocrystalline, very fine grained, apparently 

 microgranular, but the indistinct grains when highly magnified are found 

 to have a micropoikilitic structure — that is, they consist of quartz cement 

 and feldspar microlites. . There is also a much smaller amount of magnetite 

 in minute crystals, and some shreds of biotite. 



The dacite found in fragments in the breccia at the base of Mount 

 Doane (1637) is glassy and pumiceous, with abundant minute phenocrysts 

 of feldspar, quartz, and hornblende, and less biotite. These crystals are 

 nearly all idiomorphic, except when in fragments ; or in the case of quartz, 

 when rounded. The quartzes contain numerous inclusions of colorless glass 

 with gas bubble and surrounding strain phenomena. Other inclusions are 

 rare. The feldspars are plagioclase, probably labradorite. They have beau- 

 tiful zonal structure and frequent glass inclusions. Minute prisms of apatite 

 and crystals of the other constituents are sometimes included. Hornblende is 

 mon xxxi r, PT II 19 



