30 WILLIAM J. MILLER 



At the summit of Lyon Mountain the rock contains 2 or 3 per 

 cent each of hornblende, diallage, and bronzite. The feldspar 

 and quartz crystals are usually from one-fourth of an inch to an 

 inch long. Where fresh the rock is greenish gray. It weathers 

 to a light brown. A gneissoid structure is generally more or less 

 well developed, though considerable bodies of the rock may show 

 it scarcely at all. The Hawkeye granite is extensively developed 

 in Silver Lake Mountain and in the southeastern half of the great 

 mass of Lyon Mountain. 



More locally the Lyon Mountain granite grades through gra- 

 nitic syenite into quartz syenite and even into quartz diorite. 



Silexite in the granites. — The term "silexite" is elsewhere pro- 

 posed^ by the writer for any body of pure or nearly pure silica 

 of igneous or aqueo-igneous origin which occurs as a dike, segre- 

 gation mass, or inclusion within or without its parent rock. So- 

 called "quartz dikes" are included under "silexite." 



Hundreds of silexite masses were observed in the granites of 

 the Lyon Mountain quadrangle. They are most abundant by 

 far in the Lyon Mountain granite. The evidence that they are 

 not true veins is decisive. None of them ever shows crustification, 

 and many of them grade into true pegmatite. Thus at the summit 

 of Duncan Mountain and also just south of the southern summit 

 of Alder Brook Mountains, where both pegmatite and silexite 

 are grandly exhibited in the large, bare ledges of Lyon Mountain 

 granite, one may observe every possible gradation from typical, 

 moderately coarse-grained pegmatite, consisting mostly of pink 

 potash feldspar and some quartz, to about equal amounts of feld- 

 spar and quartz, to mostly quartz, with some scattering crystals 

 of feldspar, and to practically pure silica (silexite). Most of the 

 silexite contains i or 2 per cent of feldspar in the form of small 

 crystalline masses with rather blurred contacts against the silica. 

 All the facies just mentioned occur on Duncan Mountain, not 

 only in the form of dikes more or less parallel to the foliation of 

 the granite, but also often as irregular bunches or lenses crudely 

 parallel to the foliation, the original masses in the latter cases 

 apparently having been broken up, pulled apart, and the pieces 



' W. J. Miller, Science, in a number soon to be issued. 



