328 PROCEEDINGS OF THE AMERICAN ACADEMY. 



with a eutectic point between the two resulting types of mixed crystals, 

 albitc and microcHne (type V of Rooseboom). The homogeneous 

 feldspar phenocrysts, or such parts of them as are still homogeneous, 

 represent the first formed type caught in the rapidly chilled magma, 

 and now exist in an unstable condition. The cryptoperthite and 

 microperthite generally throughout the porphyries, and granites, are 

 regarded as the result of an unmixing below the transition point or 

 interval, and probably represent an approximate eutectic mixture, 

 or a true eutectic, provided that the readjustment had time to complete 

 itself. It is suggested that the prominence of the albite phase about 

 the margins of the granitic microperthite crystals, and elsewhere in 

 the rocks as an apparently late crystallization, is due, in part at least, 

 to the excess over eutectic proportions of that constituent in the 

 original mixed-crystals, and that it was set free during the unmixing 

 and thus became active during the late stages of crystallization. 

 This period of unmixing was probably that during which the aegirite 

 microliths, etc. were introduced into the feldspar of the granite. 

 This and the albite crystallizations were doubtless aided by the last 

 liquids (mineralizers) of the magma. It is thought that, at least for 

 mixtures containing as in the present case, a highly sodic member, 

 the concentrations of one feldspar in the other in the two phases of 

 the eutectic mixture, are probably not as great as those calculated by 

 Vogt, and that the true eutectic proportions ma}' depart somewhat 

 from the value deduced by him, but that this value is not much in 

 error. 



It is pointed out that following the transformations which are be- 

 lieved to have taken place during the late magmatic period, there 

 were further and more or less profound alterations of a deep-seated 

 character in the minerals of the porphyries, and to a less extent in the 

 granites. Subsequent surface alteration has, in many localities, re- 

 sulted in still further changes chiefly affecting the iron-bearing sili- 

 cates. 



The characteristics of these rocks show a close analogy in many 

 respects to those described for other intrusions of riebeckite-aegirite 

 granites, and in this respect are in general agreement with the gener- 

 alizations of Murgoci regarding this class of rocks. 



Department of Geology, 



Massachusetts Institute of Technology. 



