214 PROCEEDINGS OF THE A:MEKICAN ACADEMY. 



perthite crystal and again encroach on it from the margin. In many 

 cases where the parallel groupings of albite cut across the original 

 perthite strips, their direction of elongation and encroachment has 

 been clearly determined by the structure of an adjoining micro- 

 perthite grain. This may be in fact always the case, even when not 

 apparent. This replacing feldspar may show simple albite twinning 

 but much of it is untwinned. It is also practically free from the 

 cavities and minute inclusions characteristic of the original perthite; 

 secondary riebeckite and aegirite microliths are, however, quite 

 common in it. In sections from certain portions of the granite in 

 several localities, and particularly from the granite of Gashman's and 

 the Gold Leaf Quarries, not only may all of the above described re- 

 placements of the microperthite be seen, but much of the original 

 feldspar is seen to be replaced entirely by a very fine mosaic of feldspar 

 grains among which albite seems to clearly predominate in amount. 

 These mosaics are in part, at least, quite clearly the result of a granula- 

 tion of the feldspar replacing the original microperthite, although 

 probably the replacement and granulation were nearly or quite 

 contemporaneous processes. All gradations from the normal micro- 

 perthite crystal up to nearly or quite complete replacement and granu- 

 lation may be easily traced. The process seems to be clearly one of 

 recrystallization and albitization of the original feldspar which took 

 place, either during the last stages of the solidification of the magma 

 while the last liquids or gases were still very active, and before all 

 movement in the crystallizing mass had ceased — a protoclastic 

 structure — , or during some later recurrence of mineralizing activity 

 and movement incident to a period of dynamic or igneous disturbance 

 through which the region has passed. In either case there seems to 

 have been a considerable increase in the relative amount of albite, at 

 least locally. The first alternative as to origin appeals most strongly 

 to the writer, especially when taken in connection with a closely 

 similar change to be described later, as occurring in the feldspars of 

 the associated porphyries. 



With low magnifying powers the microperthite possesses a more or 

 less dusty appearance and in the darker varieties of the granite, in 

 rather thick sections, it is rendered almost opaque by reason of the 

 very abundant, minute particles which are scattered through it. 

 These included particles are: — 1st. — microliths of soda-hornblende, 

 and aegirite; 2nd — exceedingly minute black specks and crystals 

 (iron oxides) ; 3rd — minute indeterminate particles and cavities, 

 the latter often more or less filled with brownish or black material. 



