48 WILLIAM J. MILLER 



the gabbro was then mentioned, but no special study of the peg- 

 matites was made. In 19 18 the region was revisited, and the 

 relations of the pegmatites to the gabbro were studied in detail. 

 The gabbro nearly always occurs in the form of stocks or bosses 

 with rounded or elliptical ground plans. The stocks range in 

 length from a few rods to about a mile and in width up to three- 

 fourths of a mile. That most if not all of the gabbro stocks are 

 younger than the syenite-granite series is proved by the many 

 observed sharp contacts often crossing the foliation of the syenite 

 or granite, by many inclusions of syenite and granite in the gabbro, 

 and by sortie dikes of gabbro which extend into the syenite-granite 

 series. 



The field evidence strongly points to the mode of occurrence 

 of the gabbro as pluglike or pipelike forms with nearly vertical 

 boundaries. Much of the gabbro is of the quite typical sort, 

 consisting chiefly of basic plagioclase, some orthoclase, hornblende, 

 and hypersthene, together with almost constant small amounts 

 of ilmenite (or magnetite), biotite, garnet, and pyrite, and some 

 other accessory minerals. An ophitic texture is often well exhib- 

 ited. Many of the stocks have foliated portions, especially the 

 borders, which are hornblende gneiss or amphibolite. 



The pegmatites are mostly coarse-grained, consisting chiefly 

 of potash feldspar and quartz, but with plagioclase important in 

 some cases, muscovite and black tourmaline usually present, and 

 biotite and hornblende not rare. Practically no other minerals 

 occur. The masses of pegmatite exist in all sizes up to two hundred 

 or more feet long and fifty or more feet wide. 



Very few of the many conspicuous bodies of pegmatite of the 

 kind just described were observed apart from masses of gabbro 

 in the North Creek quadrangle, and possibly even these few may 

 not be real exceptions, because the adjacent rocks are not always 

 wholly known on account of lack of exposures. Pegmatites of 

 any kind are relatively scarce in the syenite-granite series. Since 

 most or all of the gabbro stocks are younger than the syenite- 

 granite, and since so many of the coarse, acidic pegmatites either 

 cut the gabbros or are at least genetically related to them (see 

 below), it follows that such pegmatites must be distinctly younger 



