524 PROCEEDINGS OF THE ROCHESTER MEETING 



spar which should be carefully noted. As these conditions seem to depend on the 

 degree of metamorphosing force to which the fragments may be subjected, they 

 may be called phases or stages in a progressive change, or series of changes, through 

 which feldspar crystals pass in normal petrologic history. 



These phases are three : 



The first is that of decay. A feldspar grain that is simply decayed can be identified 

 under the microscope by a peculiar flecked appearance which usually begins by 

 obscuring the margin, but finally permeates the whole grain. According to Pro- 

 fessor Pumpelly, such decayed feldspars sometimes exhibit zones of greater or less 

 alteration surrounding a core of undecayed feldspar.* The minute scales of sericite 

 are then uniformly distributed throughout the body of the crystal, or at an earlier 

 stage a kaolinization is apparent, creeping along progressively in the cleavages. 

 This decayed condition is not a metamorphic state. It is not due to a forced re- 

 crystallization, but to slow weathering. It is truly an alteration of the feldspar, 

 but it is not a regeneration. The feldspars in many of the Archean quartz por- 

 phyries exhibit this kind of alteration. 



This decay sometimes takes on a different form. The feldspar seems to change 

 wholly into an ultra-microscopic mosaic of quartz and feldspar, the little indi- 

 viduals of which are roundish but compactly adjusted together. The nature of 

 this secondary feldspar it is difficult to determine. Feldspar fragments in the 

 Ogishke conglomerate have been seen thus altered. It is possible that some faint 

 dynamic or other force, or the chemical environments, were instrumental in de- 

 termining this kind of alteration rather than the sericitic. So far as noticed, this 

 alteration affects the feldspars of the basic sediments. 



The second, or rimmed phase of regeneration, is that which has been frequently 

 noted. In orthoclase, microcline, and plagioclase the process of decay is inter- 

 rupted by a reaction resulting from dynamic or "static" metamorphism, and a 

 regenerative process begins. This seems to revive the crystallizing force which is 

 located and active in the very surface of a crystal ; and while it expels some of the 

 sericitic or epidotic particles it builds further feldspathic substances on to the 

 original grain. The new growth is fresh and glassy, making a more or less con- 

 tinuous casing on the old core. At the same time the new growth enters the old 

 grain, forming tongues or isolated small areas that polarize like the new matter 

 of the rim, giving the semi-regenerated grain a broken and mottled aspect. 

 This restoration of the old grain drives the sericite scales, the epidote, or the 

 zoisite, and all other products of the earlier decay, toward the center of the original 

 feldspar, or into groups that are prevailingly in the central part of the grain. It 

 is probable, also, that the date of definite crystallization of these impurities into 

 recognized minerals is cotemporary with this migration toward the center,,and it 

 is further probable, if the feldspar mass that immediately embraces them after 

 their migration could be specifically determined, it would be found to be of the 

 same species as that of the rim, at least in many cases. When the migration of 

 the impurities toward the center is not perfect and the entire grain is not wholly 

 renewed, it has been noted in numerous instances that the new feldspathic growths 



* Relation of secular rock disintegration to certain transitional crystalline schists. Bull. Geol. 

 Soc. Amer., vol. 2, 1901, p. 210. 



