Bulletin of the Scientific Laboratories of Denison University. 



Vol. XII. Article II. With Plates IV— V. August, 1902 



ON THE OCCURRENCE OF APLITE, PEGMATITE, 

 AND TOURMALINE BUNCHES IN THE STONE 

 MOUNTAIN GRANITE OF GEORGIA. ' 



By Thomas L. Watson, 



Aplite and pegmatite. — Careful field study of the larger and 

 principal granite areas in Georgia, by the writer, indicates the 

 general absence of true aplites therefrom. They have been 

 observed in association with the granite masses only at one 

 locality in the state. Since the border portions of the granite 

 masses are usually covered with a considerable depth of residual 

 decay and are seldom exposed, it is not possible to say whether 

 aplites as border phenomena exist, as described by Kemp, ^ in 

 some of the southern Rhode Island and Connecticut granites. 



Several aplite dikes less than six inches in width are ex- 

 posed in the quarries opened on the northwest side of the huge 

 doming ridge known as Stone Mountain, sixteen miles east of 

 Atlanta (Plate IV). Pegmatites are common associates in the 

 Stone Mountain granite and also in the other larger granite 

 masses examined in the state. They consist chiefly of coarse 

 intercrystallizations of potash (orthoclase and microcline) and 

 soda (albite) feldspars with quartz, subordinate amounts of both 

 biotite and muscovite, and occasionally red garnet and tour- 

 maline. So far as my observation goes, the feldspars in the 

 pegmatite greatly exceed in amount the quartz. The granitic 

 pegmatites are sometimes replaced, however, by those of prac- 

 tically pure quartz. In the Stone Mountain pegmatites the 

 dark minerals, mica, tourmaline, and garnet, are frequently con- 

 centrated along the central axis of the dike or vein, rather than 

 distributed through the light-colored quartz-feldspar portions. 

 Where observed the granitic pegmatites are monotonously alike, 

 and present no unusual features. 



' Reprinted from the Journal of Geology, Vol. X, No. 2, February- 

 March, 1902. Pages 186 — 193. 



^Bulletin Qeol Soc. Amer., 1899, Vol. X, p. 372. 



