238 peoceedings of the american academy. 



The Blue Hill Porphyries. 



Granite-porphyry : quartz-fdds par-por phyry . 



Distributiou; Megascopic Characteristics. — The porphyry with its 

 variations into quartz-feldspar-porphyry is the contact phase of the 

 granite magma over that part of the field enclosed within the limits 

 of the Blue Hill Reservation, and occupies, therefore, all of the more 

 elevated part of. the field. It is found outside of the Reservation 

 only in the relatively small but, nerertheless, important tract known 

 as the Pine Hill area, which lies immediately east of, and is continuous 

 with the Reservation. Here the porphyry is associated with fine- 

 granite and with a more basic phase of the contact zone, the dtirker 

 colored, rhombenporphyry. The same is true of the Pine Tree Brook 

 area and in both, it is worthy of note, that patches of the original 

 cover of Cambrian slates remain. The porph^,Ty cover does not 

 reach quite to the northern edge of the Reservation but is cut off along 

 a line that stretches across the top of Rattlesnake Hill and thence 

 rians a westerly and southwesterly direction as far as the alkaline 

 rocks extend. The areal continuity of the porphyry is broken in 

 three places by masses of aporhyolite, or felsite, as it has been called 

 for convenience in the field. The general outlines of these masses are 

 indicated on the map. Considered as later in age than the porphyries 

 and granite by Crosby, this rock is by the writer believed to be of 

 earlier consolidation and it is against the felsite that the magma 

 consolidated with some of its most interesting textural variations. 



It is quite impossible to define the extent or exact relative amounts 

 of the different phases of the porphyry, partly on account of their 

 transitions into one another, partly because of their very irregular 

 distribution and partly because of the difficulties of distinguishing 

 minor variations in heavily weathered and lichened outcrops. The 

 relative importance of the different varieties and their relation to 

 each other can, nevertheless, be made out with certainty. What is 

 here termed granite-porphyry is in areal distribution and in volume 

 the most important rock. It is, as would naturally be expected, the 

 phase that immediately overlies the granite and while in some parts 

 of the area (Pine Hill, for example) it passes gradually into the granite, 

 in others it changes suddenly into the porphyritic phase of the granite. 

 The thickness of the porphyry cover varies greatly from a few feet 

 (on Pine Hill) to about 200 feet (estimated) on Rattlesnake Hill. 



