WARREN AND PAL.\CI1E. — QUINCY PEGMATITES. 143 



per cent, reuiaiiuler 2.2 per cent. This shows that the comi)osition of 

 this rock does not probably ilepart very far from that of the j^ranite, 

 although texturally so radically ditVerent. Figure K) Plate III, is a 

 uiicrophotograph of a thin section uf the rock and will illustrate its 

 main features. 



To what extent the texture of this rock may be original, and to what 

 extent due to recrystallization under pneumatolitic conditions it is dif- 

 ficult to say. If original, we have a striking example of the strong 

 contrasts of texture which may occur in a portion of a rock-magma 

 rich in mineralizers, but not otherwise, so far as can be determined, 

 very different in chemical composition. If a product of recrystallization, 

 we see that there has been an almost complete separation of the potash 

 and soda feldspars with the consetiuent destruction of the microperthite 

 structure, as well as other minor changes. 



Chemical Composition, Se(4Uence of Cryst.a.llization, Origin. 



Without an exact knowledge of the chemical composition of the peg- 

 matites as a whole, a thing almost impossible to obtain, it is of course 

 not possible to discuss with as much certainty as we could wish, the 

 relationships that exist between the pegmatites and the granite. A 

 careful consideration of the mineral composition of the two leads, 

 however, to the conclusion that the pegmatites are decidedly richer 

 than the granite in quartz, aegirite, zircon, tluorite, the rare earth 

 mineral, parisite, and meta,llic sulphides. The central pocket in the 

 Fallon quarry pipe, which was undoubtedly originally filled with water 

 or water vapor, leads to the conclusion that the material from which 

 the pegmatite formed was also richer in water or water vapor, a con- 

 clusion in keeping with the usual idea regarding pegmatites. Of the 

 constituents present in the pegmatite in excessive amounts over those 

 in the granite magma, the water or water vapor and the fluorine appear 

 to be the only ones competent to have exerted an influence strong 

 enough to have brought about any considerable variations in the tex- 

 ture of the two rocks. And after all it appears to be true in the pres- 

 ent case, as has been suggested for other pegmatites,® that the main 

 difference between the pegmatite and its granite is irregularity of tex- 

 ture in the former accomjjanied in jiart at least by a considerably 

 greater coarseness of grain. In the present wise there is also a re- 

 markable zonal arrangement of parts as a further distinguishing mark. 



The textural relations of the minerals in the pegmatites are, up to a 

 certain point, identical with the granite, and sliow that a striking 



• Ba-stin, Jour, of Cleol., vol. IS, No. 4, I'JIO. 



