WARREN. — ALKALI-GRANITES AND PORPHYRIES. 321 



the feldspars doubtless tend to form a eutectic mixture theoretically, 

 it is highly probable that in the actual crystallization this tendency 

 may often fail of complete realization, and that Vogt's term "anchi" 

 — approximate — may be appropriately applied to the intergrowths 

 of the two feldspars as eutectics, and to the two mixed-crystal phases 

 as well. 



The phenocrysts of lavas and of the contact phases of an intrusive 

 igneous mass, like the one under consideration in the present paper, 

 grow with relative rapidity and are then frozen in the quickly consoli- 

 dated groundmass. These are precisely the conditions which are 

 favoral)le to the preservation of the crystal in the condition in which 

 it first formed, even after the temperature falls below a possible 

 transition point, provided the tendency to readjustment, which would 

 normally ensue, is not very strong, and the actual change itself is 

 hindered sufficiently by the molecular immobility of the now solid 

 crystal as well as by the relatively rigid surroundings. In other words, 

 a phase, stable at higher temperatures, is caught by the quenching of 

 the magma and rendered relatively stable at lower temperatures. If 

 a crystal in a metastable condition be long held at a temperature, at 

 or below, its transition point, or is acted upon at relatively high tem- 

 peratures by a liquid or vapor, especially if the stable phase is present, 

 it will, as is well known, generally go over to the stable phase. Pre- 

 cisel\' this thing appears to have occurred in the case of the feldspar 

 phenocrysts of the Blue Hill porphyries. Parts, perhaps almost the 

 entire phenocryst of feldspar, will be, so far as the microscope shows, 

 of entirely homogeneous structure, but through this, streaks and 

 patches are found, of cryptoperthite or very fine microperthite. As 

 we approach the margin of the crystal the same thing is observed, and 

 the crystal for an indefinite, short distance from the margin consists 

 of a perthitic intergrowth. The streaks and patches in the inner 

 parts of the crystal are believed to be due to an unmixing under the 

 action of long continued heat, accompanied probably by the action 

 of heated vapors or solutions permeating the rock. The marginal 

 microperthite may be due to the same cause, but it seems more probable 

 that at the time when this grew the temperature had fallen to a 

 point below the transition interval, and the two phases separated more 

 or less completely, forming the crypto- or microperthite; the crypto- 

 perthite here, and in general, representing a structure resulting under 

 conditions less favorable to a free unmixing than the microperthite. 

 It is to be noted that the generally widespread and clearly marked 

 recrystallization of the phenocrysts in the porphyries is, it is believed, 



