50 WILLIAM J. MILLER 



hill one mile south-southeast of the Glen. The gabbro stock, 

 which is one-fourth of a mile long, shows a number of pegmatitic 

 satellites, and that it is younger than the surrounding granite is 

 proved by several inclusions of the latter. Figure 7 illustrates 

 the relations of several facies of the pegmatite to the gabbro near 

 the northeastern edge of the stock. Crystals in any of the peg- 

 matite seldom reach lengths of an inch. The acidic pegmatite 

 with quartz carries biotite, and it grades perfectly into the rather 

 acidic pegmatite without quartz but with hornblende. In both 

 of these facies of the pegmatite the chief feldspar is oligoclase to 

 oligoclase-albite. Contacts against the gabbro are usually rather 

 sharp. The more basic pegmatite, which consists chiefly of basic 

 plagioclase and hornblende with some magnetite, shows no sharp 

 contact against either the acidic pegmatite or the gabbro. That 

 the various facies of the pegmatite formed while the gabbro magma 

 was still in a distinctly fluid condition is considered to be proved, 

 not only by the presence of well-defined inclusions of the pegmatite 

 mostly parallel to the magmatic flow-structure of the gabbro, 

 but also by the strong tendency of the fluxion structure of the 

 gabbro to wrap around the inclusions and to conform to the sides 

 of the large body of pegmatite. Quite clearly the molten gabbro 

 intruded and more or less broke up the already consolidated peg- 

 matite which had probably developed as a magmatic segregation 

 mass. The basic pegmatite may have developed somewhat later 

 than the other, as suggested by the lack of anything hke sharp 

 contacts against the gabbro, but even this pegmatite formed while 

 the gabbro was still fluid enough to develop a fluxion structure 

 alongside the pegmatite. 



Near the middle western edge of the same gabbro stock a 

 pegmatite mass, with a maximum width of twenty-five to thirty 

 feet and an exposed length of about one hundred feet, shows rela- 

 tions notably different from those on the opposite side of the stock 

 above described. Some of the features are brought out by Fig. 8. 

 Toward one side of the general body of pegmatite a mass of practi- 

 cally pure silica (silexite) has imbedded in it many rather imperfect 

 crystals of black tourmaline which mostly range in length from one 

 to ten inches. This small mass is surrounded by very coarse pegma- 

 tite made up chiefly of potash feldspar and quartz with a little tour- 



