444 THE CRYSTAL FALLS IRON-BEAEING DISTRICT. 



with feeble double refraction, which seems to be quartz. Between crossed 

 nicols the groundmass is almost isotropic, and it is by no means improbable 

 that certain reddish patches here and there may realh^' be glass. Little crys- 

 tals of magnetite are abundantly scattered through the groundmass, and are 

 often arranged in parallel curving lines, very suggestive of the flowage 

 lines brought out on the surface of weathered rhyolites by the ferruginous 

 stains. In many sections the groundmass includes minute lath-shaped 

 plagioclase feldspai's, much altered and with indistinct boundaries, which 

 are often arranged in jDarallel lines. The groundmass also is g-enerally 

 sprinkled with little irregular grains of epidote and calcite. 



In this groundmass are included in variable combinations and propor- 

 tions much larger crystals and grains of common hornblende, actinolite, 

 biotite, ottrelite, calcite, ilmenite, epidote, and zoisite. Of these biotite, 

 calcite, ilmenite, epidote, and zoisite are the most constant and abundant. 



Biotite is present in all or nearly all of the thin sections. It is always 

 brown, and is characteristically developed in stubby individuals, very thick 

 for their basal dimensions. These individuals are large and lie scattered 

 through the slides. They frequently inclose portions of the groundmass. 

 The mica cleavage most frequently stands across the cleavage of the roek. 

 In many of the darker-colored schists, however, biotite plates intermediate 

 in size between the large porphyritic indi^'iduals and the small chlorite 

 plates of groundmass are present in large numbers, constituting a sort of 

 secondary groundmass. These are generally aligned with the cleavage 

 of the rock and are sometimes gathered in bands, but in color and stubby 

 habit are similar to the phenocrysts. 



Ilmenite in brownish-black prismatic sections is a common constituent. 

 It usually lies at random through the slide. It incloses the quartz and 

 epidote g-rains of the groundmass. Epidote and zoisite are exceedingly 

 abundant, often in well-formed crystals. ' Many of the epidote and zoisite 

 individuals contain darker colored inner nuclei, the nature of which is 

 uncertain. In some cases the nuclei are irregular in shape and have 

 the characteristic pleochroism of epidote, but are more strongly colored 

 than the surrounding zones. In other cases they have sharp crystal 

 boundaries, isomorphous with epidote, are brown in color, and inclose 

 grains of magnetite; these may be allanite. The nuclei are too small, 

 however, for determination. Grenerally they do not extinguish exactly with 



