Art. XI. 1 Watson, The Lcopaniile of North Carolina. 227 



proportion in all of the .section.s studied. Wiieii they form 

 complete spheres, which is rarely the case, they usually exhibit 

 somewhat irregular rag_<;"ed peripheries, and further show usu- 

 ally between cross nicols a very indefinite black cross. The 

 form of the grains in the typical micro-granitic areas of the 

 groundmass is sharp and allotriomorphic to partially idiomor- 

 phic. The principal groundmass minerals are feldspar and quartz, 

 with much light-colored mica, and an occasional inclusion of 

 prismatic apatite and zircon. Irregular minute grains of iron 

 oxide are scattered through the sections, and stained areas from 

 manganese and iron oxides, forming the dark spots and pencils 

 in the hand specimens, occur. The thin sections are character- 

 ized by the complete absence of ferro-magnesian minerals. 



Feldspar is apparently in largest quantity, and is composed 

 of both potash and plagioclase species. Occasional grains 

 of microcline are recognized which show the characteristic mi- 

 crocline twinning. The unstriated feldspar grains so strongly 

 resemble quartz that it is impossible in many cases to distin- 

 guish them without the application of optical tests. Optical 

 tests show the plagioclase to be albite — a circumstance entirely 

 confirmed by the chemical analysis of the rock given below in 

 the table of analyses under I, in which only the barest trace of 

 lime is indicated, with soda in large amount and in excess of 

 the potash. Some of the plagioclase exhibits polysynthetic 

 twinning according to the albite law. and at times assumes lath- 

 shaped forms. The feldspar substance is generally fresh, but 

 the individual grains are usually rendered dark by abundant, 

 closely crowded, minute, dark, dust-like particles, the identity 

 of which could not be made out. 



Quartz is of the usual kind and presents no noteworthy 

 features, further than its occurrence in small mosaics of inter- 

 locking grains, which occupy at times distinct areas in some of 

 the thin sections. 



Light-colored mica, tinged a faint yellow, is very generally 

 distributed through the sections, in the form of irregular minute 

 shreds, groups, anjd aggregated masses, the folia of which are 

 at times imperfectly arranged radially about a common center. 



