242 PROCEEDINGS OF THE AMERICAN ACADEMY. 



movements along the contact, in part portions of the finer grained 

 contact quartz-porphyry. The zone of coarse porphyry near the 

 extreme eastern end of the felsite mass is from three to four feet 

 wide. Further west the band broadens out somewhat. Going to- 

 ward the granite the groundmass becomes gradually coarser in grain, 

 the dark mineral appears in distinct spots resemljling more the horn- 

 blende of the granite and the rock passes gradually into the granite 

 which, however, retains, for an indeterminate distance, a porphy- 

 ritic texture and this, as has been already noted, is always an indica- 

 tion of a near approach to the granite-porphyry. 



Small cognate Xenoliths of a dark colored rhoml)en porphyry 

 similar in all respects to those described later under the heading of 

 "Xenoliths" (p. 275) are found, sometimes quite abundantly in the 

 coarsely porphyritic zone, almost up to the actual contact. These 

 are also found, but more sparingly, in the granite-porphyry and granite 

 immediately succeeding. 



At the eastern end of the felsite, the breadth of the entire porphyry 

 zone is, so far as it can be determined, only about 15 ft. Going west 

 it broadens rapidly (30 ft.). This width holds nearly to the top of 

 Pine Hill and then still further broadens (50-100 ft.) while at the 

 same time the coarse-porphyry type disappears, and porphyry of the 

 Rattlesnake Hill type replaces it. 



In many parts of the Blue Hills the phenocrysts of the porphyry 

 are quite inconspicuous, the rock having a relatively dense and non- 

 porphyritic appearance. The microscope shows, however, that this 

 texture has been produced by a breaking of the phenocrysts through 

 movements in the mass before crystallization had stopped and Ijy 

 the very general recrystallization of the constituents, particularly the 

 feldspar. 



Those varieties of the porphyry in which the phenocrysts are less 

 numerous and in which the groundmass is relatively dense might 

 be classed as Paisanites ^^ but the greater part of the porphyry appears 

 to the writer to be too abundantly phenocrystalline and its ground- 

 mass too coarsely crystalline to be properly classed under this name, 

 although in mineral and chemical composition they are closely related 

 to the original paisanite. 



In closing the description of the field characters of the porphyry 

 one more important feature must be noted. Throughout large por- 

 tions of the higher elevations of the hills, numerous, angular inclusions 



33 Osann, Tschermaks Min. u. Pet. Mitt. XV Band. 



