322 PROCEEDINGS OF THE AMERICAN ACADEMY. 



an evidence of the generally unstable condition of the feldspar pheno- 

 crysts as first formed from the magma. As still further hearing on 

 this question we may note that the smaller and later formed pheno- 

 crysts are largely microperthite, and that the feldpsar which developed 

 in the groundmass of the porphyries, in part attaching itself to the 

 older phenocrysts, is all microperthite. This part of the feldspar 

 content of the rock obviously solidified at lower temperatures than 

 the phenocrysts, and, if our theory is correct, formed the region lying 

 below the transition interval. 



When we come to the coarse-granite we find that all of the feldspar 

 is a relatively coarse microperthite. In that part of the magma 

 which eventually formed the granite a A'ery long cooling period ob- 

 tained, during which there would be ample opportunity for the establish- 

 ment of more perfect equilibria in general. The mixed crystals of 

 soda-potash feldspar which first formed would pass through the 

 transition interval, but, in sharp contrast to conditions obtaining in the 

 porphyry magma, there would be no supercooling, and enough time 

 for the readjustments to take place. The new phases did not move 

 far, but rather withdrew from each other, and being still closely similar 

 crystallographically, finally took up the relative positions in which we 

 now find them; viz. — they formed an intimate, crystallographic 

 intergrowth. 



Regarding the composition of the soda-potash-feldspar eutectic, 

 there is still, as is to be expected, both on account of the peculiar 

 physical properties of the feldspars alluded to above, as well as on 

 account of the character of the available evidence bearing on the 

 question, much uncertainty. The approximate compositions of the 

 feldspars, as calculated from the analyses of the Quincy-Blue Hill 

 rocks, show some divergence from the figures worked out by Vogt. 

 The limiting values for the concentrations of one feldspar in the other, 

 as given by that author, appear to the MTiter as being open to some 

 question. In the present rocks the sodic member, so far as can be 

 told, appears to be a very pure albite, likewise the potash member 

 seems to be a highly potassic microcline. It would appear, if the 

 writer's deductions are correct, that the concentrations of the two 

 feldspars in the mixed crystal phases may be lower, at least for the 

 highly alkalic feldspars, than is indicated by the figures of ^ ogt. 

 That the eutectic composition lies in the general region indicated by 

 him appears probable. It is of course true that Vogt's conclusions 

 include more calcic plagioclases than are concerned in the present case, 

 and it seems not improbable that with the presence of more of the 



