230 PROCEEDINGS OF THE AMERICAN ACADEMY. 



presence of abundant, minute prisms, scales, fibers and irregularly 

 shaped pieces of the blue-hornblende scattered through the ground- 

 mass, as well as fibers distributed along the cleavages and the direction 

 of perthitic intergrowth in the feldspar; and in the nearly or complete 

 disappearance of the aenigmatite. It is not probal)le that the dark 

 porphyry originally differed from the grey in chemical composition and 

 the present differences are undoubtedly due to alteration acting along 

 zones or streaks in the porphyry mass as a whole. The alteration is 

 different from that produced by purely superficial alteration, and is 

 believed to be a result of deep-seated alteration (connected probal)ly 

 with late magmatic conditions) in the same M'ay that the corresponding 

 streaks of dark-grey granite are. 



Characters of the Granite-Porphyry elsewhere in the ^irea. — Both 

 the light and dark Aarieties have a very extensive development 

 throughout the Blue Hills, l)ut microscopic study of a large numl^er of 

 specimens from many points shows that there have been considerable 

 changes effected in the various minerals beyond those which are 

 thought to have developed during the late magmatic stage or in one 

 immediately following it. These changes may perhaps have been 

 effected during periods of profound geologic disturbance through 

 which the region has passed, but the writer is inclined to think that 

 they are in the main but a continuation of the modifications described 

 as occurring in the Rattlesnake Hill porphyry, developed during the 

 period subsequent to the consolidation of the porphyry, before the 

 granite magma below had completed its crystallization and was still 

 capable of giving off mineralized vapors, and perhaps also before all 

 movements as a result of upward pressure of the mass beneath had 

 ceased. Upon these are often superimposed the effects of superficial 

 decay although these are limited to a thin surface layer. It thus 

 happens that many exposures afford specimens which depart more or 

 less widely in the details of the structure and composition from the 

 normal type, and not a few in which only the remnants of the original 

 structures remain. 



Perhaps the most striking modifications thus effected are those 

 shown by the feldspar phenocrysts. The changes in the original 

 feldspar substance, believed to have been largely brought about 

 during the latter part of the crystallization period or immediately 

 following it, have been described for the normal porphyry, and are 

 to be seen more or less strongly developed everywhere in the porphyry, 

 although they are, to a greater or less extent modified by subsequent 

 and more general alterations. So varied are the details of these 



