WARREN. — ALKALI-GRANITES AND PORPHYRIES. 273 



north side of Rattlesnake Hill) where apparently again the magma 

 had an opportunity to differentiate. They also appear in the granite 

 dikes which cut the porphyry cover from the underlying granite, as 

 shown near Slide Notch and in the Pine Tree Brook areas. 



It is noteworthy that in the thick mass of granite-porphyry as ex- 

 posed at Rattlesnake Hill, and generally in the same rock which is 

 characteristic of the higher levels of the contact, that cognate xenoliths 

 are of very rare occurrence. The same is true of the fine-granite. 

 Inclusions of the fine grained, little or undifferentiated contact phases, 

 as has been noted earlier, are abundant at the contacts and many 

 angular masses, undoubtedly of the same origin, are to be found in 

 many of the porphyry ledges throughout the Blue Hills proper. 



Although there is a strong localization of the xenoliths in the regions 

 indicated, they are also common throughout the granite mass as a 

 whole. The contrast in texture and mineral composition is, to be 

 sure, not as marked in the latter class of occurrences, but it is the belief 

 of the writer, founded on extensive observation about the quarries, 

 that there is not a cubic yard of granite in which some more or less 

 marked variation in texture cannot be found on careful examination. 

 Single surfaces of one or more square yards may be obtained which are 

 practically free from noticeable ^'ariations in grain, and many such are 

 to be seen in the finished blocks at the quarries, but a careful inspec- 

 tion of the sides and back of the blocks will show one or more patches 

 differing from the normal more or less sharply. Quarrying has pene- 

 trated into the granite to a depth of over 300 ft. and the xenoliths are 

 still in evidence, although the writer feels pretty certain that in depth 

 they are either somewhat less numerous than higher up, or at least they 

 are less sharply distinguished in texture. 



The Pine Hill area in West Quincy offers the best opportunity 

 to study the xenoliths which are associated with the deeper contact 

 zones. The darker colored, fine grained and usually porphyritic type 

 is common in the granite-porphyry almost up to the aporhyolite 

 contacts and is also found in the porphyritic-granite immediately 

 underlying it. In this phase of the granite generally, and particularly, 

 in the northern part of the area in the immediate neighborhood of the 

 heavy developments of the rhombenporphyry, this type of xenolith 

 is very abundant, but it is associated with types that are lighter in 

 color and with a more granitoid groundmass as well as with some 

 which are feebly or nonporphyritic. In the more normal granite 

 which is exposed over the eastern and southeastern parts of this par- 

 ticular area but which is always prone to pass into the porphyritic 



