WARREN. ALKALI-GRANITES AND PORPHYRIES. 291 



Some of these have a length of nearly one hundred feet by somewhat 

 less in width and are stated by Crosby to show a very constant strike 

 of N. 80 E. with a dip, S. 80-85, the metamorphism by the granite 

 not having been sufficient to obliterate the true bedding of the slate. 

 This constant orientation of isolated pieces of the slate is held by 

 Professor Crosby to indicate that they are — " roots of a once con- 

 tinuous body of slate." 



The granite remains coarse up to within an inch of the slate and then 

 is fine-grained to the contact, nor was the granite necessarily rendered 

 finer because of chilling, but may have developed simply a finer grain 

 induced by direct pliysical contact with the slate surfaces. The slate 

 is very dense, hard, and somewhat, though not very highly, meta- 

 morphosed, and shows no evidences of having received any additions 

 from the magma with which it was in contact. The absence of ex- 

 treme metamorphism certainly does not argue in favor of any excess 

 of heat nor of \olatile products in this part of the magma, a point 

 that is of importance in any consideration relating to the method by 

 which the magma came into its present position. 



The absence of the contact phases, which are elsewhere developed 

 between the granite and the older rocks, and the present higher 

 elevation of the slate-granite contacts on North Common Hill relative 

 to other parts of the field where the contact phases are strongly de- 

 veloped — Pine Hill for example — makes it necessary to assume, that 

 either these contacts were originally much deeper seated portions 

 of the contact and since have been elevated, or that the slate patches 

 represent sunken blocks frozen in the granite, and that somewhere 

 above the present plane of erosion there once existed a cover of the 

 contact porphyries. The constant orientation of isolated blocks of 

 slates over so considerable an area, and the presence of a well defined 

 fault contact lying but a little way to the north with a probal)le up- 

 throw of the igneous rocks relative to the sediments *^, points very 

 strongly to the correctness of Crosby's conclusion that the slate here 

 represents remnants of the original slate contacts. 



Pegmatite "Pipes." 



Three pegmatitic masses having the form of elongated, pipe-like 

 bodies occur in the granite of North Common Hill, Quincy. These 

 are remarkable for their structure and crystallizations and have been 

 made the subject of an extended description by Professor Charles 



45 See G. F. Loughlin, op. cit., p. 29. 



