106 HENRY S. WASHINGTON 



growths of feldspar and quartz are rather common. Colored 

 components are rare, those most often met with being olive- 

 green hornblende and brown biotite, while colorless diopside is 

 less frequent. A few specimens carry titanite and magnetite, 

 while apatite is seldom seen. These aplites are occasionally 

 coarser-grained, and become microgranites, when they resemble 

 closely the granites proper,, except that the crystallization is on a 

 smaller scale, and colored minerals are rare. 



The most interesting aplitic dike found is one cutting a gran- 

 ite exposure near Bass Rocks, Gloucester. It is the same mass 

 as that mentioned previously with enclosures of orbicular syenite 

 and diorite, and the aplite cuts granite and enclosures impartially. 

 The dike is only 6-7 cm wide, and is notable on account of being 

 compound in an anomalous way. The borders, about 2 cm on 

 each side, are of fine-grained white microgranite speckled with 

 small, black biotite and hornblende flakes. This shows under 

 the microscope the usual microgranitic structure and characters. 

 Extending down the center is a band, about 2 cm in width, of a 

 dense, almost aphanitic light gray aplite, which shows under a 

 lens only very minute black specks. This in thin section is a 

 very finely granular aggregate of round quartz and alkali- feld- 

 spar anhedra, with here and there small shreds of green horn- 

 blende and pale brown biotite. 



The junction between the two facies of the rock in the hand 

 specimen is slightly irregular but sharp. Under the microscope 

 it is also seen to be fairly sharp, but the quartzes and feldspars 

 of the borders show a tendency to granularity and passage into 

 the aplite which is suggestive of crystallization out of one magma 

 and not that due to two injections. This idea of differing crys- 

 tallization in one magma is also supported by the uniformity with 

 which the aplitic band sustains its central symmetrical position 

 with reference to the two sides. The highly anomalous charac- 

 ter of the dike will have been noticed, in that the borders, con- 

 trary to the usual rule, are more coarsely crystalline than the 

 central portion. If we adopt Judd's 1 view of the composite dikes 



1 J. W. Judd, Q. J. Geol. Soc, Vol. XLIX, p. 536, 1893. 



