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PROCEEDINGS OF THE AMERICAN ACADEMY. 



by the mavked dilTerences in texture of the two rocks) may determine 

 quite surpri.sing- (Hfferences in tlTC minor essential minerals. 



In order to see if there are chemical differences between the granite- 

 porphyry and its contact phases two partial analyses were mg-de: 

 one (No. 11), on as fresh a specimen as could be obtained from the 

 porphyry-aporhyolite contact just south of Rattlesnake Hill. This 

 specimen came from the immediate contact and under the mici'oscope 

 showed small phenocrysts of quartz and feldspar embedded in a very 

 fine groundmass rich in aegirite microlites. The second specimen 

 (No. 12) came from about a foot from the porphyry-aporhyolite con- 

 tact near its eastern end on Pine Hill, West Quincy. The specimen 

 is of the coarsely and profusely porphyritic type with feldspar pheno- 

 crysts of large size, and is the type characteristic of a deeper level of 

 the contact, with these analyses is given in part that of the normal 

 granite-porphyry of Rattlesnake Hill (No. 10). 



^Yhile the specimens were somewhat altered the analyses indicate 

 that there has been little, if any, real differentiation in the porphyry. 

 The almost exact reversal in the relations of ferrous to ferric iron of 

 the extreme contact phases, in comparison with the rock of the main 

 mass of porphyry, with but a slight gain in the total iron oxides, is 

 striking and in keeping with the strong development of aegirite in the 

 contact rock, and may point to stronger oxidizing conditions near the 

 contact, though in view of the variation in these oxides in the granite 

 itself, its significance is doubtful. 



