WARREN. — ALKALI-GRANITES AND PORPHYRIES. 251 



changes as a whole that it would be impossible to fully describe them 

 and only brief description of some of the more characteristic and com- 

 mon types will be attempted. The accompanying microphotographs 

 (Figures Nos. IV, V, VI, VII, Plates 1 and 2) will serve, perhaps better 

 than the descriptions, to furnish an idea of the appearance of some of 

 the types of modified phenocrysts. One of their most characteristic 

 features is that great numbers of them are crossed by bands or streaks 

 which are optically continuous with the marginal parts of the pheno- 

 cryst and with it form a sort of mesh enclosing a heterogeneous mix- 

 ture of feldspar material. Theses "streaks" nearly always show a 

 distinct central division line representing an original crack now sealed, 

 and extend partly or entirely across the crystal in slightly curved and 

 often ramifying lines. Along the central division line there is usually a 

 narrow streak of albite material, and on either side of this for a variable 

 distance the material shows a faint microperthitic structure as do the 

 marginal parts of the crystal with which it is in fact continuous. 

 Further away from these bands, mingled with remnants of the original 

 feldspar, may be more coarsely developed microiDcrthite, or separately 

 crystallized albite and mici'ocline. Frequently these separate crystals 

 are orientated parallel to the original feldspar or stand normal to the 

 edges of the "streak"; again they are situated quite at random. 

 They may take the form of rather short laths or are entirely irregular 

 in outline. As a rule albite appears to be more abundant than would 

 be expected if the changes were concerned wholly with the rearrange- 

 ment of the albite and microcline of the original feldspar and there 

 may have been a later introduction of albite. The microperthite 

 can be easily recognized as a rule by the presence of minute specks, 

 cavities, etc. in the potash member just as in the granite micro- 

 perthite. Wherever this has been broken or replaced, the specks, 

 etc. are absent, but needles of blue amphibole occur, sometimes with 

 aegirite. Both aegirite and amphibole are commonly developed 

 along the original division line in the "streak" and abundantly on 

 either side of the streak as a whole. It appears that the cracks served 

 as channels along which solutions acted, introducing probably some 

 material from without and effecting changes for a short distance on 

 either side in the original feldspar — a more distinct development 

 of the albite and potash members — thus rendering such portions of 

 the feldspar, like the outer margin, relatively more stable and per- 

 mitting them to persist more or less intact while the less stable interior 

 of the phenocrysts underwent a considerable and often a nearly or 

 complete recrystallization and replacement. The question of the 



