468 THE CRYSTAL FALLS IRON-BE AEING DISTJBICT. 



sheets of paper. They are the cross sections of the mica coatings on the 

 cleavage planes. 



The inspection of this rock in the hand specimen and in the ledge 

 leads to the same conclusion — that it is an intermediate or an acid lava, a 

 porphyrite, or a porphyry that was squeezed until it became schistose and 

 sheared until it became fissile. 



Under the microscope the feldspar phenocrysts, though much decom- 

 posed and filled with inclusions of quartz, muscovite, and other decomposi- 

 tion products, are well enough preserved to exhibit in some places twinning 

 striations. The greater portion of the phenocrysts are untwinned. The 

 twinned material borders the grains, fills in cracks between Carlsbad twins, 

 and, is irregularly distributed through the untwinned material, occurring 

 more particularly in those places where the decomposition of the original 

 feldspar is most complete. The twinned feldspar is fresher than the 

 untwinned variety. This fact and the manner of its distribution indicate a 

 secondary origin for it. The quartz phenocrysts are rare. They present 

 their usual characteristics. 



The groundmass in which the phenocrysts lie is a fine-grained aggre- 

 gate of biotite, quartz, and plagioclase. The biotite is a greenish-brown 

 variety. It occurs in large plates arranged in parallel position and in 

 small flakes occupying the same parallel position and lying between the 

 quartz and the plagioclase grains. The banding noticed in the hand speci- 

 men is due to the arrangement of the large biotite flakes in bands. These 

 are separated from each other by bands of quartz and plagioclase that are 

 free from the large biotites, though they contain innumerable small flakes 

 of this mineral. Only when a porphyritic crystal lies in the way of the 

 bands do these depart from their uniform directions. Here they bend 

 around the phenocrysts, leaving on both sides of them little triangular areas 

 in which the components are much finer grained than elsewhere in the rock. 



The light-colored components are quartz and plagioclase. These min- 

 erals are- in small grains that appear to be intercrystallized in the manner 

 of the secondary aggregate that constitutes the fine-grained matrix of many 

 o-reenstones, of the aporhyolites, and of other rocks that have sufl^ered 

 intense metamorphism. The quartzes are nearly always crossed by strain 

 shadows and the fresh clear plagioclase by interrupted and bent twinning 

 bars. 



