40 WILLIAM J. MILLER 



quadrangle, however, even these minerals are very rare or absent. 

 Aside from water vapor, then, these later pegmatites must have 

 been mostly relatively free from mineralizers. 



Figure 2 shows one of the later pegmatites with fairly sharp 

 contacts cutting across a ledge of granite which contains inclusions 

 of silexite and eyes or inclusions of older pegmatite. 



Near the top of the hill two miles east of Goldsmith a dis- 

 tinctly later pegmatite dike not only cuts across the fohation of 

 the granite but also sharply cuts earher-formed lenslike masses of 

 both pegmatite and silexite. 



Just south of the southern summit of Alder Brook Mountains 

 several dikes of ordinary pink pegmatite grading into practically 

 pure silexite cut the granite in all directions without very sharp 

 contacts. 



A fine ledge clearly exhibiting the relations of earlier pegmatite 

 and silexite to later pegmatite occurs on the little hill one-fourth 

 of a mile northwest of the mouth of the eastern of the Two Brooks 

 near Goldsmith. Thin-bedded Grenville quartzite and gneiss 

 are mostly highly injected with medium-grained granite, pegma- 

 tite, and silexite. This combination of rocks is locally much 

 contorted, owing to magmatic movements, and the pegmatite 

 and silexite, especially the latter, are much broken up and kneaded 

 into the mass, thus proving that this pegmatite and silexite origi- 

 nated before the solidification of the granite magma. Ordinary 

 pegmatites cut across this ledge of mixed rocks in true dikelike 

 form at various angles, and they are not broken up, thus clearly 

 indicating their distinctly later origin, that is, after the solidifica- 

 tion, or at least almost complete solidification, of the granite 

 magma. 



Before leaving the consideration of the later pegmatites, the 

 occurrence of magnetite, hornblende, and pyroxene in many of 

 them should be briefly discussed. One or more of these minerals 

 may be observed in the later pegmatites of the Lyon Mountain 

 quadrangle in many places, but almost invariably only where the 

 pegmatites are known to be directly associated with an older 

 hornblende gneiss or gabbro, though not all the pegmatites thus 

 associated contain one or more of the minerals mentioned. The 



