WARREX. — ALKALI-GRANITES AND PORPHYRIES. 243 



of a dense to very finely crystalline rock rather sparsely sprinkled 

 with minute black specks of hornblende are found. These are usually 

 small rarely measuring more than a few inches across. When altered 

 they are of a light-grey or light, bluish-grey color and usually show a 

 few small phenocrysts of feldspar. They belong to the alkaline 

 series, and seem to be essentially identical with the porphyry in com- 

 position, but appear to differ in texture from the inclusions in the 

 brecciated portions of the porphyry before alluded to. These corre- 

 spond closely in many respects to paisanite. 



Microscopic characters of the Porphyry: — The granite-porphyry 

 from Rattlesnake Hill in the eastern part of the Reservation will first 

 be described, for it is here that it is perhaps best exposed and where 

 fresh material for sections and chemical analysis can be most easily 

 obtained. 



In thin-section the light greenish-grey porphyry, which is consid- 

 ered the normal type, is seen to consist of abundant phenocrysts of 

 feldspar (ca. 40%), quartz (ca. 12%) (see accompanying table, col- 

 umn I), areas of hornblende and pyroxene (mostly aegirite), in part as 

 distinct grains but largely in the form of poikolitic intergrowths with 

 the groundmass, all embedded in a fine groundmass of microperthite, 

 quartz, hornl^lende, and aegirite. Accessory aenigmatite, magnetite, 

 hematite, zircon, fluorite and occasional calcite and astrophyllite are 

 also present. 



Table of Rosival estimates of Quartz and Feldspar Pheno- 

 crysts IN Porphyry. 



100.0 100.0 100.0 100.0 



I Unaltered Granite PorphjTy, Rattlesnake Hill, average measurements 

 of two sections. 



II Quartz-feldspar-porphyry, ledge S. of Administration Road, S. of Wam- 

 patuck Hill, — 11 feet from aporhyolite contact. Made on one large section. 



III Same, 3" from same contact. 



IV Coarsel}^ porphyritic phase of contact porphyry. Contact east end 

 of aporhyolite Pine Hill, West Quincy. Average of two extra large thin- 

 sections. 



Among the phenocrysts, the feldspar greatly predominates. Before 

 modification by breaking and later recrystallization, the feldspar 



