42 WILLIAM J. MILLER 



the pegmatite containing bunches of green, monochnic pyroxene 

 and hornblende which look like inclusions up to five inches across. 

 The pyroxene developed by recrystallization of some of the horn- 

 blende of the hornblende gneiss during the process of pegmatization 

 for all stages, from bunches and lenses of pure hornblende, to 

 mixtures of hornblende and pyroxene not sharply separated, to 

 pure pyroxene, occurs in the pegmatite. Also, in thin section every 

 stage in the transformation of the hornblende to pyroxene has been 

 observed. This pegmatite carries some magnetite and in one por- 

 tion scattering single crystals of hornblende up to one inch long, and 

 in another portion crystals of pyroxene up to one-half of an inch long. 



ApUte dikes. — Aplite dikes are locally developed often in con- 

 siderable numbers in the coarse or Hawkey e type of granite. It 

 is a striking fact that nowhere in the quadrangle were aplite dikes 

 observed in the typical Lyon Mountain granite which contains 

 such a profusion of pegmatites and silexites. Possibly this is 

 because the Lyon Mountain granite is itself locally rather aplitic 

 in character. Since the Hawkeye granite contains relatively few 

 pegmatite and silexite masses, such could seldom be observed in 

 their relations to the aplites in single outcrops. Few if any aplites 

 occur in the granitic syenite or quartz syenite. It should here 

 be recalled that the field evidence strongly points to development 

 of the Lyon Mountain granite, Hawkeye granite, granitic syenite, 

 and quartz syenite from a single intrusive magma in the Lyon 

 Mountain quadrangle. 



Nearly all the aplites of the quadrangle are in the form of more 

 or less well-defined dikes which are very uniform in composition, 

 consisting of white or light-gray potash feldspar, microperthite, 

 and quartz with almost no accessory minerals. In a few cases 

 dark minerals occur in moderate amounts. 



Most of the aplite dikes by far lie more or less perfectly parallel 

 to the foliation of the coarse granite, but a considerable number 

 cut across the foliation at various angles. Not uncommonly the 

 cross-cutting dikes are branches of those which are parallel to the 

 foliation. Contacts of the aplites against the granite are rarely 

 sharp, though the transition generally takes place within an inch 

 or two. Some of the aplite dikes, more especially the wider ones, 



