MINERALOGY AND PETROLOGY. 975 



Gneiss frequently passes into] 

 mica schist. 



7. Gneiss, which frequently passes into mica schist, has the composition and 

 microscopic character of igneous granite. It is frequently intersheeted with mica 

 schist over wide areas with great regularity, such that there is no possible way of 

 reasonably explaining it, other than to ascribe it to a sedimentary origin and hence 

 an original clastic structure. The straight lines of this stratification sometimes 

 continue visibly without much deviation for the distance of half a mile or more. 



This regrowth of feldspars (and other minerals) has not always been inter- 

 preted in this way. When seen in a plainly clastic rock such marginal growths 

 have been simply referred to as " enlargements," and when in a crystalline or igneous 

 one, they have been considered either as secondary growths in the original magma 

 prior to consolidation, as zonal increments after the partial resorption, or as renewals 

 of feldspars after partial fusion by a basaltic contact. The indication that springs 

 from our study, however, points to the essential sameness of these renewals, whether 

 in clastic or crystalline rocks, and to the continuance of this manner of crystallization 

 through all grades of development from the most faintly metamorphic clastic to the 

 most perfectly recrystalline igneous. 



At the same time, when such regenerated feldspars have been seen in granites, 

 with the " kaolinic " impurities that cloud them driven to or toward their central 

 areas, they have sometimes been supposed to be instances of grains centrally altered 

 through weathering, or some other agency. But several reasons have been given in 

 the previous portions of this chapter for discarding. that idea (page 942). It might 

 be stated further that the weathering of a feldspar crystal would begin at the surface 

 and remove its alkaline bases (No. 25Wbis); it would not recrystallize them as 

 silicates in the centre of the crystal. Again, this weather effect, if it be such, apper- 

 tains to the rock masses at all depths, and is not attributable, in Minnesota, to weather- 

 ing since the glacial epoch. It is reasonable to ascribe the decay to weathering in 

 Archean time, followed by a powerful reconstructing force by which the alkaline 

 elements once disengaged by weathering were forced into silicated combinations 

 such as muscovite, hornblende and epidote. 



As to the effect of dynamic agents, there is no question that they have had 

 great influence in producing the regeneration per se, they could have had no effect 

 in causing the decay. They could not centralize the muscovite in the orthoclases 

 without a profound molecular rearrangement, resulting, when carried to completion, 

 in a re-formation of the whole rock. Such force could have promoted the formation 

 of the secondary silicates (muscovite, epidote, etc.), but could not have separated the 

 elements of those minerals from their former combinations. They might break the 

 crystals, thus facilitating the access of destructive atmospheric agents. It requires, 

 however, both dynamic force and moisture to carry forward the changes which the 



