284 PROCEEDIXGS OF THE AMERICAN ACADEMY, 



the contact zone. For example, while the xenolith analyzed (17) 

 differs sharply from the fine-granite of the Ruggles Creek type in 

 having aegirite instead of riebeckite, it does bear a very close resem- 

 blance in this respect, as well as others, to the fine-granite and granite- 

 porphyry covering the coarse granite with its pegmatite dikes on the 

 knoll just east of the R. R. track, N. E. of the Pine Hill area. If we 

 disregard minor variations, the resemblances, of the porphyritic 

 xenoliths to the rhombenporphyry and to various phases of the granite- 

 and quartz-feldspar-porphyry characteristic of different parts of the 

 contact zone on the one hand, and that of the finely granitic feebly- 

 or non-porphyritic xenoliths to the fine-granites on the other hand, 

 are striking and have an important bearing on the origin of the xeno- 

 liths as a whole. 



The Aporhyolite. 



Distrihution . — This rock has a relatively large development in the 

 area. It occurs in at least three separate masses (see general map), 

 which collectively cover several square miles. The first and largest 

 occupies the southwestern part of the Pine Hill tract, and may be 

 connected with the large area of aporhyolite which occurs within the 

 Blue Hill Reservation lying to the south of Rattlesnake Hill, extending 

 southward to the borders of the Reservation where it is lost beneath 

 the great swamp in northern Braintree. According to Professor 

 Crosby it again appears at one point beyond the southern border of 

 the swamp. The area covered by this occurrence is certainly not less 

 than two square miles. The second mass in size is that found lying to 

 the north and northwest of Fox Hill. The third is a relatively smaller 

 mass occupying a portion of the top of Hemingway Hill. 



The Pine Hill aporhyolite (see special map) begins a few hundred 

 feet east of the summit of the hill. It here forms a nearly north-south 

 contact with the porphyry phase of the granite intrusion for about fifty 

 feet. The supposed apophysis of aporhyolite described by Crosby 

 as cutting across the contact-porphyry at this point, appears on 

 microscopic study to be in reality only the very fine (felsitic), extreme 

 contact phase of the porphyry, the contact here being irregular in 

 direction for several feet. From the north-south contact just referred 

 to, the aporhyolite mass extends to the west, and broadening out, one 

 contact runs in a west-southwesterly direction for a few hundred feet 

 when it is entirely lost beneath a heavy mantle of drift, but ledges of 

 the rock extend continuously nearlv to Willard Street. The other, 



