3l8 EDSON S. BASTIN 



on the other hand, the contact relations are wholly different, the 

 change from pure granite to pure sediments taking place gradually 

 through a transition zone of contact metamorphosed and injected 

 sediments two to three miles in width. These transition zones include 

 a great variety of rocks, slate, schist, injection gneiss, flow-gneiss, 

 diorite, diabase, pegmatite, and granites of various textures, all asso- 

 ciated in the most irregular manner so that it is impracticable to delin- 

 eate them separately in ordinary geologic mapping. In western and 

 southwestern Maine these transition zones are much broader than in 

 the Rockland quadrangle and contain larger amounts of pegmatite 

 and granite gneiss and smaller amounts of basic igneous rocks. 



The contrast between the sharpness of certain granite contacts 

 observed in the Bluehill region and the very gradual transitions 

 observed in the Rockland quadrangle and farther southwest seem to 

 be best explained on the hypothesis that the broad injected zones 

 represent portions of the "roof" of granite batholiths, whereas the 

 sharp contacts represent the sides of similar batholiths. The charac- 

 ter of the rocks which are found in the two types of contacts lends 

 support to this view. The more ready escape of water gas and other 

 gases and their dissolved substances upward than laterally may 

 explain the great abundance of pegmatite in the broad transition zones, 

 inasmuch as the presence of such gases is believed to be the most 

 important factor in the development of pegmatitic texture. It is 

 a reasonable supposition that basic differentiation from the granitic 

 magma would also be more rapid upward than laterally, and the 

 abundance of diabase and diorite in certain of the transition zones 

 may thus be accounted for. The hypothesis is also in accord with 

 the low temperatures at which certain portions of the pegmatites 

 appear to have crystallized, in comparison with the temperatures of 

 crystallization of normal granites, and accords with the presence of 

 numerous dikes of very fine-grained granite, some so fine as to be 

 rhyolitic, in certain of the contact zones, and their absence about the 

 sharper contacts. 



Summary. — Field and laboratory studies of the Maine pegmatites 

 indicate that all are in a broad way contemporaneous and are geneti- 

 cally related to the associated granites. 



External conditions, though locally having some slight influence, 



