42 THE CRYSTAL FALLS lEOiS^-BEAEING DISTRICT. 



the individual sliows mieroclinic striations. The other part was untwinned, 

 and near the center of the phenocryst, bisected by the Carlsbad twinning 

 plane, was found a rectangular plagioclase crystal. 



The microcline is usually the best crystallized feldspar in the ground- 

 mass, and also by far the freshest. In the few cases in which it was 

 observed in contact with plagioclase, the latter molded it, and is therefore 

 older than the microcline, which in its turn is older than the orthoclase. 

 In one case a microcline individual showing the lattice structure over a 

 portion of its surface possesses no twinning- lamellae in another portion, the 

 twinning lamellae fading until they totally disappear. Thus no sharp 

 delimitation is apparent between the twinned and untwinned portions of the 

 individual. 



In most slides all the feldspars are much altered, but even in those in 

 which the microcline is fresh the plagioclase and orthoclase always show 

 alterations, the plagioclase altering most easily and usually being so changed 

 that it is with difficulty that one can recognize the twinning lamellae. Hence 

 some of them may have been taken for the nonstriated orthoclase. In an 

 early stage of the alteration of the feldspars minute dark ferrite particles 

 which impregnate them are hydrated, and this gives the feldspars a more or 

 less distinctly red tinge. In a more advanced stage of alteration, muscovite 

 and a little epidote-zoisite are produced. Another alteration of the feldspar 

 is always associated with marked pressure phenomena, and hence is pre- 

 sumed to be the result, partially at least, of dynamic action. This is the 

 partial or complete granulation of the feldspar and the production from 

 that mineral, with the addition from other sources of the iron and magnesia 

 necessary, of secondary white mica and quartz, and some biotite. It is 

 highly possible that some of the small limpid grains considered to be 

 secondary quartz are really an acid feldspar. Orogenic movements are 

 also indicated by the bending of twinning lamellae, and were probably the 

 partial cause of the twinning. 



Biotite occurs in plates, and as a rule shows better-developed crystals 

 than does the feldspar, though it frequently occurs in decidedly ragged 

 flakes. It is strongly pleochroic, showing absorption in the following colors: 

 Pale straw yellow to yellowish brown, for rays vibrating perpendicular to 

 cleavage, to very dark chocolate brown and greenish brown for those par- 

 allel to cleavage. In the case of the biotite showing a greenish color this 



