324 PROCEEDINGS OF THE AMERICAN ACADE:MY. 



molecular mobility and openness of structure, if we may use such a 

 term in this connection, to favor the introduction of extraneous 

 material, aegirite, etc? Does not also the presence of the innumeral^le, 

 minute vesicles in part, at least, date from this period of change? It 

 is of course possible that the aegirite, etc. represents material that 

 was originally held in solution in the feldspar material, and that it 

 separated out during the unmixing of the original crystal. No micro- 

 liths of aegirite nor of any other mineral appear in the homogeneous 

 parts of the phenocrysts in the granite-porphyry, and this alone is be- 

 lieved to be very strong evidence that they made their appearance 

 during the unmixing and probably by introduction from sources out- 

 side the feldspar crystals themselves. This has, of course, no con- 

 nection with those microlithes and larger crystal grains, such as the 

 riebeckite, which are connected with later alterations in the rocks. 

 In this connection it is perhaps worth noting that the pyroxene and 

 amphiboles may also undergo transformations of which we have as 

 yet no definite information. 



Summary. 



The alkaline granitic magma of Quincy and the Blue Hills was 

 intruded as a small batholithic mass, later than the middle cambrian, 

 but earlier than the late carboniferous period. The method of in- 

 trusion is believed to have been one of "stoping." The magma as a 

 whole is believed to have stoped its way upward to a relatively high 

 elevation in the pre-existing formations, and a considerable portion of 

 it reached a position very near the surface. As a result, the upper 

 portions consolidated to highly vitreous rocks, while somewhat lower 

 portions formed thick masses of porphyritic, crystalline rocks, — 

 quartz-feldspar and granite-porphyries. The extreme upper portions 

 were perhaps more siliceous and richer in volatile constituents, which 

 in large part may have escaped ; lower portions consolidated without 

 substantial differentiation or change. As an incident to intrusion, 

 the earlier consolidated portions were much broken, engulfed in the 

 magma beneath, and against these, as against the original contacts, 

 the residual magma consolidated with a varying degree of chilling, 

 according to position. At considerably lower contact levels now 

 exposed in the eastern part of the field, the magma consolidated with 

 a marginal zone of fine-granite as the contact phase. Here also the 

 first formed rock was intruded by, broken up, and to some extent 



