236 ANNALS NEW YORK ACADEMY OF SCIENCES 



In mineral composition, they vary from true granites and aplites in the 

 case of the medium-grained varieties to nearly pure quartz veins in the 

 case of the pegmatitic types. In the pegmatites, the greatest variation in 

 mineral composition is found. They range from coarse-grained granite 

 to pure quartz. In addition to the orthoclasc, phigioclase (either alhite or 

 oligoclase), quartz, muscovite, biotite and black tourmaline which are most 

 frequently present, a great many other more unusual minerals are some- 

 times available for the collector. Among these, the following have been 

 identified by various mineralogists : amphibole, apatite, antunite, beryl, 

 chrysoberyl, columbite, cyanite, cyrtolite, dumortierite, garnet, ilmenite, 

 iolite, monazite, pinite, titanite, uraconite, uraninite, uranotile, xenotime 

 and zircon. 



In texture, these pegmatites are often very coarse. Feldspar crystals 

 may reach a length of several feet, as in the case of the Bedford dikes, 

 and many of the other accompanying minerals will have a correspondingly 

 large size. 



Some interesting structural features are also developed in the pegma- 

 tites at times. Yeiy coarse pegmatite may often be associated with me- 

 dium-grained granite in the same dike or sheet. The gradation from the 

 one into the other may be a gradual one or it may be quite abrupt. Where 

 such relations occur between granite and pegmatite, the fonner appears 

 to have been intruded first and to have been followed closely by the latter, 

 sometimes before the first had had an opportunity to completely solidify. 

 Often the granitic phases in the case of the intrusive slieets show an 

 original gneissoid structure. 



A banded structure very similar to that sometimes seen in true veins is 

 also developed (PI. IX, Fig. 2). In instances, this structure is due to the 

 growing inward from the ^^■a]ls of crystals of some of the minerals present, 

 very often the muscovite. At other times,, it is due to tlie progressive in- 

 crease in size of the crystals of the mineral constituents from the walls 

 inward. It may also be brought about when the intrusion of a granite or 

 aplite is closely followed by the injection of pegmatite along the same 

 fissure and before the foi-mei- has had an opportunity to cool and com- 

 pletely solidify. 



In the above description, it has been assumed that the pegmatites are 

 of igneous origin, a view now quite generally accepted by geologists. ^^ It 

 is thought that they represent the final products of crystallization of rock 

 magmas. 'J'hey are the "mother liquoi*" so to speak, containing the bulk 



*3 W^. C. BrOggkk : Syenit I'l'gmatitgiinso der siid norwejiischen Aus>it iind Nepbelin- 

 syenite. Zeits. f. Kryst. u. Miner.. Vol. 10, pi). 2ir>-285. 1800. 



Joseph P. Iddixgs : Igneous Rocks. Vol. I, pp. 278-270. 1900. 



Alfred IIarkeh : The Natural Ilistory of Igneous Rocks, pp. 203-209. 1909. 



