PEGMATITE, SILEXITE, AND APLITE OF NEW YORK 43 



exhibit good magmatic flow-structure foliation. No case of an 

 aplite dike grading into pegmatite was observed, though one ex- 

 ample of gradation to silexite was noted. The aplite dikes usually 

 vary from one or two to several feet wide and ten to twenty-five 

 feet long, but some are considerably larger. 



The field evidence strongly indicates that all or nearly all the 

 aplites developed during a very late stage of the consolidation of 

 the Hawkeye granite magma, as shown by the common parallelism 

 to the foHation and the lack of very sharp contacts. There is 

 no evidence, such as that above presented, that any of the aplites 

 formed as early as the earliest of the pegmatite and silexite masses, 

 and there is positive evidence that some, and strong probability 

 that many, of the latest pegmatites were intruded after the aplites. 

 Considering the time then during which the pegmatite and silexite 

 masses developed, the aplites are intermediate and the length of 

 time for their development was distinctly shorter. Descriptions 

 of a few selected examples will serve not only to show the nature 

 of the evidence upon which the foregoing conclusions are based, 

 but also to bring out certain other features of interest. 



A fine display of aplite dikes may be seen on the ridge one-half 

 of a mile southeast of Hawkeye post-office. Contacts against the 

 typical coarse granite are not sharp, but the transition takes place 

 within an inch. The dikes show all widths up to three feet. Most 

 of them roughly follow the foliation of the granite, but some cut 

 across at low angles and a few at high angles. One dike Hes at 

 right angles to the foliation for about fifteen feet and then grad- 

 ually swings into parallelism with it. A few rods east of this 

 locality some lenses of aplite a few feet long occur with somewhat 

 sharper contacts against the granite. This suggests an earlier 

 development of aplite masses, either as segregation masses or as 

 dikes, which were pulled apart and somewhat strung out in the 

 form of inclusions in the still moderately fluid magma. On the 

 ridge one-sixth of a mile northeast of the first-named locality an 

 aplite dike twelve to eighteen inches wide and exposed for forty 

 feet cuts diagonally across the foliation of the coarse granite with- 

 out sharp contacts. Almost exactly in its middle there are two 

 masses of silexite fifteen feet long and a few inches wide, with no 



