REGENERATION OF CLASTIC FELDSPAR 525 



are not of the same species as the old grain. The crystal orientation is usually the 

 same in the new as in the old, but not always. The albite mark may be contin- 

 uous from the old into the new, or it may be interrupted. The carlsbad twin may 

 be ignored by the new growths. If both new and old are orthoclastic, one may be 

 deformed and the other not deformed orthoclase. The new feldspar may be 

 twinned like microcline when the old is not. The new feldspar may be polysyn- 

 thetically twinned when the old is not. Hence it appears that the new growths 

 may vary in a great variety of ways in crystalline structure, both in specific 

 designation and chemical composition, from the original grain. Such semi-renewed 

 clastic feldspars are common in the Archean sediments of Minnesota. They have 

 been described in the schists and conglomerates of the Taconic region of New 

 England. There is no doubt that they can be found in many other places where 

 clastic materials have been metamorphosed. 



In the third phase of feldspar enlargement the secondary growths are much ex- 

 tended. The original forms of the feldspar grains are nearly or wholly lost. The 

 boundaries are jagged and even grown into the spaces between adjoining grains. 

 Sometimes the new growths have surrounded adjacent grains of other minerals. 

 This change in the feldspars is coordinated with similar transmutations in all the 

 associated minerals. The result is the formation of a granitic texture by the re- 

 crystallization of the entire mass. In this case there can be no doubt of the unity 

 of the feldspar species, whether in the new growths or in the remaining cores. 

 The old cores have been thoroughly saturated with the new feldspathizing solu- 

 tions. The sericitic or zoisitic particles remain, grouped at the centers of the 

 feldspars ; the clear and fresh growths spread outward in all directions, giving 

 place, however, at their margins to other minerals, but extracting from the rock 

 everywhere every atom of the chemical elements that can be seized on to promote 

 their own development. The rock thus passes to a normal granite or diorite or 

 other massif, according to the mineral composition. 



This regrowth of feldspar is apparent, so far as examined by the writer, in nearly 

 all the metamorphic and massive acid rocks. It has not always been interpreted in 

 this way. Owing to the imperialistic sway of an old dogma as to a pronounced dif- 

 ference in origin between clastic and acid massive rocks, these regrowths have not 

 been allowed to have their legitimate effect in petrological studies. When they 

 have been seen in plainly clastic rocks they have been referred to simply as acci- 

 dental, marginal enlargements, and when in crystalline or igneous rocks they have 

 been explained as secondary growths in the original magma prior to consolidation, 

 or after effusion as zonal increments after partial resorption, or as renewals of feld- 

 spar after partial fusion by a basaltic contact. There seems to be, however, no 

 essential difference in the quality of this change from first to last. The difference 

 is one of degree. 



The inference that is to be drawn from a consideration of these successive phases 

 in the metamorphism of feldspar relates to the genesis of granite and its allies. If 

 it be true, in one instance only, that a granite can be shown to originate in this 

 way, it is indicative of a law for the generation of all granites. If it be found that 

 in many cases the same facts are grouped so as to point to the same law, it is suffi- 

 ciently demonstrative of the universality of that law to warrant its adoption and 

 incorporation into the petrology of the crystalline rocks. 



