38 WILLIAM J. MILLER 



mouth of Alder Brook. One mass of pegmatite a few feet wide 

 perfectly exposed for twenty feet contains orthoclase crystals up to 

 three or four inches long, and its boundaries against the granite 

 are sharp. A very evident magmatic flow-structure follows the 

 details of the very irregular boundary on either side within six 

 inches of the pegmatite, but one or two feet out the foliation is 

 practically straight. Evidently this t}^ical pegmatite developed 

 unusually early, probably by segregation, perhaps as early as some 

 of the first-formed silexite, that is to say, when the granite magma 

 still possessed a considerable degree of fluidity. 



Later pegmatites. — As regards their mode of occurrence these 

 later pegmatites are in most respects quite ordinary. They cut 

 across the granite foliation at all sorts of angles, and hence they 

 must have developed after complete, or at least almost complete, 

 solidification of the granite which contains them. They are true 

 dikes, generally coarser grained and usually larger than the older 

 pegmatite masses above described. Many of them are from one 

 to ten feet wide and from ten to fifty feet or more long. They 

 rarely if ever exhibit graphic intergroWths. Contacts against the 

 granite range from very sharp to moderately sharp, those with 

 very sharp boundaries probably having developed in the solid, 

 relatively cooler granite, while those with less sharp boundaries 

 probably were intruded into essentially solid but relatively hot 

 granite. Most of these dikes consist largely or wholly of pink or 

 white potash feldspar and quartz; some contain hornblende or 

 pyroxene, or both; some contain little or no quartz; some are 

 almost pure quartz (silexite); a few contain scapolite, and many 

 contain magnetite in amounts ranging from i to 40 or 50 per cent. 

 With few exceptions these later pegmatites, like the earlier ones, 

 do not contain any other minerals as well known in pegmatites in 

 general. As pointed out below, the hornblende, pyroxene, and 

 magnetite were almost certainly absorbed by the pegmatites from 

 the old, invaded dark gneisses and gabbro. Apatite in small 

 amount and many specks of titanite occur in some of the peg- 

 matized zones of the granite in the main workings of the iron mine 

 at Lyon Mountain village, and in somewhat greater amounts in 

 the older workings to the east. Throughout the Lyon Mountain 



