WARREN. — ALKALI-GRANITES AND PORPHYRIES. 323 



lime feldspar molecule, different conditions of equilibrium come into 

 play, and that for a certain concentration of anorthite, complete 

 crystal miscil)ility may not obtain, but only partial miscibility, and 

 that such feldspar systems belong to type V only of Roosebooms' 

 classification. 



The unmixing of a prcA'iously homogeneous mixture is believed to 

 offer also an explanation of the occurrence of albite about the margins 

 of the microperthite grains in the granite, and also in part, of the pres- 

 ence of strongly marked albitization of some of the original feldspar 

 in the porphyries. Assuming that the albite present in the soda- 

 potash mixed-crystal was in excess of the eutectic proportions stable 

 at lower temperatures, we should expect that, in the process of read- 

 justment which would take below the inversion point above postu- 

 lated, that some of the albite phase would be set free and that it would 

 be forced out upon the final crystallization of the microperthite. 

 Much of it would be expected to attach itself directly to the albite 

 exposed along the margins of the microperthite crystals, while some 

 of it would grow alongside as separate crystals or even migrate to 

 more distant parts of the rock. The same thing would happen in the 

 case of the feldspar growing in the groundmass of the porphyry, but 

 there would lie present the metastable, earlier formed phenocrysts, 

 which wovild be readily eff'ected by the liquors rich in albite, thus 

 proflucing the albitization described as so commonly observed in the 

 porphyries. This theory relie\'es us of the necessity of calling upon 

 the underlying magma to supply albite bearing solutions in the con- 

 siderable amount necessary to furnish all of the later albite in the 

 porphyry. It does not, however, l^y any means exclude the prob- 

 ability, that the underlying, crystallizing magma has furnished some 

 part of the albite. 



Devdopment of Microliths in the Feldspar. — The writer has ex- 

 pressed the opinion in this paper and elsewhere ^^, that the microliths 

 of aegirite found so abundantly in the feldspar of the granite were not 

 products of earlier crystallization, but were of later origin. During 

 the readjustments in the feldspar to meet changed conditions of 

 equilibrium have we not a most favorable time for the introduction of 

 those minute crystallizations of a mineral which is known to have been 

 growing during the later period of crystallizing activity? During the 

 readjustments within the feldspar, would we not have the degree of 



88 Warren & Palache. Pegmatites of the Quinoy Granite, etc., oi^. cit. 

 p. 145. 



