128 UNIVERSITY OF COLORADO STUDIES 



in motion. Again the mineral appears in minute flakes apparently 

 pseudomorphous after pyrogenetic biotite, since aggregates of these 

 flakes have the same outline as the biotite crystals in other specimens. 

 This secondary biotite must have formed after the magma had come 

 to rest. Ordinarily the biotite is yellow-brown in basal sections and 

 gives a uniaxial interference figure. In the freshest specimens the 

 flakes often suggest resorption by the presence, on the border, of minute 

 grains of an opaque substance probably magnetite. Often the biotite 

 is completely replaced by epidote and chlorite with a small amount of 

 magnetite, less often by muscovite and chlorite with some magnetite. 

 Muscovite phenocrysts are few in number and are, at least in part, 

 secondary after biotite. 



The quartz phenocrysts are invariably corroded and embayed. It 

 is quite probable that in the last stages of phenocrystic development 

 resorbed silica from the quartz phenocrysts furnished the acid necessary 

 to the formation of the more sodic feldspars which form the outer zone 

 of the plagioclase phenocrysts. Inclusions of groundmass are present 

 in nearly every quartz examined. Occasionally a part of the phenocryst 

 has been torn from the parent crystal and the interspace filled with 

 groundmass. Liquid inclusions are abundant, sometimes in rows, 

 oftener irregularly distributed. 



Groundmass. — This is composed of small grains of unstriated feld- 

 spar, minute flakes of muscovite, scattering grains of magnetite, occa- 

 sional zircons and, in a few specimens, small apatites. In addition 

 there are in some sections minute prismatic crystals strongly resembling 

 apatite, but too small to identify. The magnetite is in irregular grains 

 in the dacite near Sunshine Canyon, but in octahedrons and dodeca- 

 hehrons in the Lefthand variety where it is probably primary. In the 

 altered rock secondary calcite is abundant. The only apparent differ- 

 ence between the dacite north of Lefthand Canyon and that near Sun- 

 shine Canyon, in addition to the absence of flow-structure from the 

 former and the presence of euhedrons of magnetite, is the somewhat 

 coarser texture of the same variety. 



Accessories. — In addition to the isolated crystals noted above small 

 zircons are enclosed in all the phenocrystic minerals with the possible 



