WARREN. — ALKALI-GRANITES AND PORPHYRIES. 311 



parallel growth of microperthite to the already existing feldspar, 

 forming a relatively narrow rim about the larger ones, but a much 

 broader one about the smaller and later phenocrysts. This marginal 

 feldspar includes many quartz and aegirite grains, indicating its con- 

 temporaneous age.^^ The first hornblende formed small massive 

 crystals suggesting a slight priority of growth, and as its crystalliza- 

 tion progressed, it enclosed poikilitically the quartz and feldspar of 

 the true groundmass, as well as some of the smaller and later feldspar 

 phenocrysts. The greater part of the aegirite, excepting the crystals 

 of distinctly earlier age, is distributed through the groundmass in the 

 form of minute crystals or clusters. The aenigmatite appears to have 

 followed about the same plan of growth as the hornblende. 



In the long period during which the granite beneath was solidifying, 

 the porphyry must have remained at a temperature not far below its 

 crystallizing interval, and during this period certain mineral and 

 textural changes took place. The most obvious ones were those 

 connected with the recrystallization of a part of the phenocrystalline 

 feldspar and its albitization, fully described in the earlier part of this 

 paper and to be referred to again. There were doubtless other changes 

 among them, possibly those resulting from some coarsening and modi- 

 fication of the texture by "sammelkrystallization," as it has been 

 termed by F. Rinne.^^ 



If we examine the phases of the granite-porphyry which are nearer 

 the granite in the field, we find the same order of progression, but the 

 pause is perhaps less sharply marked, the grain of the groundmass is 

 coarser indicating a longer period of cooling, during which there was 

 time for a more nearly granular type of texture to develop. In the por- 

 phyritic phase of the granite, which comes next in order, we find that 

 the pause is almost or quite obliterated and that the groundmass is 

 barely distinguishable from the rest of the rock. Passing now to the 

 coarse-granite, we cannot doubt that the first crystallizations were 

 substantially the same as in the porph\Ty. The larger amount of 

 quartz relative to the feldspar in the granite suggests that the quartz 

 preceded the feldspar in the beginnings of its crystallization. In 



67 In the extreme contact phases the rims about the feldspars are very nari'ow 

 or impreceptible. This is to be expected, owing to the greater degree of super- 

 coohng resulting in a greater impediment to free molecular movement in the 

 highly viscous residual liquors. It should be noted that in some parts of the 

 porphyry a later rim is also found about the quartz phenocrysts, as might be 

 expected. 



68 Fortschrift fur Mineralogie, Krystallographie u. Petrographie, 1, p. 209, 

 et seq. 



