F. II. Lahee — Metamorphism and Geological Structure. 457 



wide, or in the form of radiating aggregates known as ' plumose 

 mica.' The garnets seldom attain dimensions greater than J 

 inch. 



— Relations between the pegmatite phase and the granite 

 phase. — From the foregoing remarks it is evident that the 

 composition of the pegmatite closely resembles that of the 

 Boston JN r eck granite — so closely, indeed, that if for no other 

 reason, we should be inclined to regard both as genetically akin. 

 There are other grounds, however, for this assumption. In the 

 first place, pegmatite may traverse the granite in the form of 

 dikes or as regular patches and streaks. Especially in the latter 

 case, the two rocks grade into one another so insensibly that 

 no boundary can be set between them. These pegmatite bodies 

 would seem to be of the residual type, as described above. In 

 the second place, the granite sometimes becomes pegmatitic 

 toward the Carboniferous schists, so that its contact zones, 

 instead of being finer in texture, may be coarser than its normal 

 phase, and its apophyses into the schists may be, and in fact 

 generally are, pegmatitic. This is marginal pegmatite. Along 

 the coast south of Watson's Pier, granite and pegmatite may 

 be seen thus intimately associated with blended contacts, the 

 effect often being that of schlieren on a large scale. South- 

 ward, the pegmatite phase becomes less important ; northward, 

 the granite phase becomes less important. The last evidences 

 of the granite are seen in narrow streaks in a large pegmatite 

 dike on the Bonnet (small projection of western coast in 

 C:14, %. 1). 



— Relations of the pegmatite phase to the Carboniferous 

 sediments. — Those pegmatite dikes which cut the Carboniferous 

 formation may be many feet or less than an inch in thickness. 

 On Boston ^eck they are very numerous and very large, and 

 the schists occupy relatively little space. Exposures on the 

 Bonnet, for instance, are chiefly of the schists which, however, 

 are intersected by iive or six large dikes and many little ones. 

 Still farther north, on Barber's Height \'Loc. 9, B : 12), pegma- 

 tite is rarely encountered. 



Great thickness does not necessarily imply great length ; 

 for intrusives twenty or thirty feet across may thin out and 

 come to an end (at the topographic level) within a space of 

 twenty-five feet.' On the other hand, apophyses from the 

 larger dikes may run with nearly a uniform width of only five 

 or six inches for distances of several scores of feet ; but they 

 commonly end by tapering to a point within a few yards of 

 their parent body. Sometimes the thickness may alternately 

 increase and decrease, even to such a degree that an intrusion 

 appears in cross-section as a succession of short, discontinuous, 



