GRANITES OF NORTH CAROLINA 401 



individuals are frequently filled with crystals, or grains of other min- 

 erals, arranged without reference to one another or to their host. 

 Hardly without exception, the inclosing mineral is a potash feldspar, 

 and the included minerals are quartz and plagioclase, with an occa- 

 sional shred of biotite. In the phenocrysts of the porphyritic granites 

 this order is frequently reversed, and biotite is often inclosed in larg- 

 est amount. Variation is from several scattered included grains in 

 the feldspar to hosts fairly filled with the inclusions. The inclosed 

 quartz grains are rounded in outhne, drop-hke in form, while the 

 striated plagioclase may vary from similarly rounded grains to lath- 

 shaped forms. 



Because of its occurrence in devitrified glasses, WilKams' ascribed 

 in some cases a secondary origin to the very abundant micropoikilitic 

 structure in the ancient acid lavas of South Mountain, Pennsylvania, 

 and Maryland. There is no evidence for regarding the structure in 

 the Carolina and Georgia granites of other than primary origin — a 

 fact which aids, as I interpret it, in formulating the order of separa- 

 tion of the feldspars and quartz from the magma. When viewed in 

 connection with the granophyric structure described above, the two 

 microstructures furnish conclusive evidence of the overlapping in the 

 period of crystalHzation in the potash and plagioclase feldspars and 

 the quartz in the southern granites. A tendency was noted in many 

 of these sections toward maximum development of the two structures 

 in the same thin section. This seeming association was by no means 

 a constant feature, for many sections in which granophyric inter- 

 growths were present indicated the entire absence of the micropoi- 

 kilitic structure. 



INTERSECTING DIKES AND VEINS. 



Genetically the intersecting materials are of two kinds, true dikes 

 and true veins. These contrast quite strongly in some instances, 

 and in all cases they show differences to some degree in both texture 

 and composition. 



BASIC IGNEOUS DIKES. 



Dikes of basic igneous rocks, principally diabase or its altered 

 form, were observed penetrating the granites in most of the impor- 

 tant areas in the state. A majority of these were noted from surface 



I G. H. Williams, American Journal of Science, Vol. XLIV (1892, 3d S.), p. 482. 



