MICROSCOPIC DESCRIPTION 437 



be the result of hemimorphic structure. Most appear completely filled 

 with liquid, but some show a minute, fixed dark bubble. A few green- 

 ish scales of hornblende are also sometimes included in the mineral. 

 From feeble double refraction the interference colors between crossed 

 nicols rarely reach reddish orange of the first order in a thick section. 

 Extinction parallel, but angles from 5 to 12 degrees or more are some- 

 times noted along division planes crossing this mineral and the contiguous 

 hornblende; c = a. In convergent light, imperfect axial figures are ob- 

 served ; double refraction positive. In slide C a peculiar alteration of 

 zoisite was noticed, into minute, colorless, micaceous scales. In D the 

 proportion of zoisite arose to perhaps 20 per cent, but all in coccolitic 

 form, as if crushed up into rounded and oval granules. 



The groundmass filling the interstices of the hornblende blades pre- 

 sents a limpidly clear mosaic of closely fitted angular grains of feldspar 

 and quartz, about 0.5 to 1.5 millimeters in length. 



The feldspar consists chiefly of plagioclase, with a few inclusions of 

 hornblende, zoisite, hematite, and rare, minute prisms of colorless zircon, 

 with high relief. Near the margin of the thin-section occasional traces 

 of cleavage occur. Between crossed nicols many grains present dual 

 twinning ; others are polysynthetic after the albite law, and a few also 

 show the pericline twinning. Twinning lines are sometimes bent along 

 a fracture through a grain, wavy extinction is common, and occasional 

 instances of zonal banding are found. Determination of maximum 

 extinction angles in sections normal to the albite twinning gave about 

 25 degrees, indicating labradorite (AbsAnJ ; the grains are entirely 

 allotriomorphic. 



The presence of orthoclase is suggested by a very few grains, without 

 twinning, which possess zonal banding, wavy extinction, and approxi- 

 mately rectangular cleavage. In slides Q and Gr feldspar is nearly or 

 quite absent. It is also rare in if, where epidote occurs in abundance 

 in pale yellow, pleochroic grains and rods, 0.1 to 0.4 millimeter in 

 length. In J", epidote is still more abundant. 



Quartz occurs in very variable proportion, almost absent in some slides 

 ((t), and up to 30 per cent of the volume in others (C). The quartz 

 grains are surprisingh^ clear, with even fewer inclusions than those of 

 feldspar. Fluid cavities are absent. Some grains are hypidiomorphic, 

 showing two or three sides of a hexagon. Both quartz and feldspar ofi'er 

 all evidences, at least in their present form, of late development in occu- 

 pation of the interspaces of the rock. Wavy extinction is general in 

 both minerals, and often assumes a form which is not described, I be- 

 lieve, in the books. The darkening begins all around, at the contour of 

 the grain, and, with continued rotation between the nicols, progresses 



LXI— Bull. Geol. Soc. Am., Vol, 14. 1902 



