270 PROCEEDINGS OF THE AMERICAN ACADEMY. 



As noted, the magnetite, or more probably ilmenite, is almost always 

 closely associated with the augite. Apatite is quite abundant and is 

 found in or about the augite and also in the feldspar. The quartz is 

 mostly confined to the groundmass where it forms extremely irregular 

 masses moulded in between the other minerals. Its amount is usually 

 very small but in some of the less highly porphyritic types it is more 

 abundant and, as will be noted later, it begins to be more plentiful 

 both in the form of phenocrysts and in the groundmass, in xenoliths 

 of the porphyry further removed from the contacts, and particularly 

 in the xenoliths, such as are abundantly developed in the granite of the 

 Pine Hill area. 



In some portions of the less porphyritic and more siliceous types 

 a green to greenish-blue, alkali hornblende makes its appearance, 

 poikilitically enclosing the groundmass feldspar. In these, also, the 

 pyroxene in part shows by its green color the presence of the aegirite 

 molecule, and there is an obvious passage toward the more acid phases 

 now represented largely by the cognate xenoliths to be considered 

 later. 



A study of the porphyry-slate contact exposed on the Sawcut Notch 

 road shows that the porphyry forms a chilled contact against the 

 slate. The feldspar phenocrysts become somewhat smaller in size 

 and less numerous as the contact is approached, while at the same 

 time the groundmass becomes very fine. The actual contact seen in 

 thin section shows the two rocks in sharp contact. The slate shows a 

 slightly coarsened grain in some of its minerals and there is a patchy 

 development of biotite plates immediately about the contact within 

 the slate. On the whole, while the slate is hard and very dense, the 

 contact metamorphism appears to have been relatively slight. 



Chemical Characters. — The great variation in the texture of this 

 rock rendered the selection of a material for analysis difficult. Several 

 specimens, taken from the old road-metal quarry in the northern part 

 of the Pine Hill tract, furnished the best material, although the altera- 

 tion of the rock even here, is greater than desirable for chemical study. 

 The sample used was assembled from good sized fragments broken 

 from specimens which represented the principle variations in texture 

 noted, varying from profusely and coarsely porphyritic ones to those 

 finer in grain and only moderately porphyritic. The analysis, there- 

 fore, is believed to represent very fairly the average composition of 

 this differentiate of the Quincy-Blue Hill magma. Tlie average 

 of duplicate analyses is given inider column 13. 



