GEOLOGY OF GRAVES MOUNTAIN 219 



/ 



Mineral Genesis. 



Genesis of the minerals of tlie unusual association noted at Graves 

 Moiuitain cannot be considered entirely solved until more detailed work in 

 the field has been accomplished. ^The quartzite in which the minerals 

 occur affords evidence of both anamorphic and katamorphic changes. 

 Anamorphism is shown chiefly in the production of foliation, the formation 

 of certain characteristic heavy minerals, and recrystallization. Crushing, 

 fracturing, and brecciation of the quartzite, and the occurrence of quartz 

 veins are structures characteristic of the zone of fracture or katamorphism. 

 No intrusions of igneous rocks of any kii^d have been observed at Graves 

 Mountain, although granites and basic igneous types are known at several 

 localities some miles away. It is not known whether the igneous rocks are 

 older or younger than the quartzite composing the mountain. The age 

 of the rocks has not yet been determined but reasoning from similarity of 

 lithologic types of widely separated areas it seems not improbable that the 

 Graves Mountain quartzite will prove to be of Cambrian age. No evi- 

 dence is available at present, for regarding the minerals as having formed 

 from the effects of intrusions of igneous rocks. 



Dr. Genth regarded the minerals of this locality, except rutile and 

 quartz, as secondary products derived from the alteration of corundum. 

 He says:* 



The same association of cyanite, rutile, pyrophyllite and lazulite in an arenaceous 

 sandrock is found at Graves Mountain, Lincoln County, Georgia, and although as 

 far as I am aware,„corundum has never been found at this place, there can be very 

 little doubt that at this locality also all these species except rutile and quartz owe 

 their existence to the_ former presence and subsequent alterations of corundum. 



Not only has corundum never been found at Graves Mountain but the 

 authors have discovered no evidence for regarding the minerals as having 

 been derived from such alteration. 



From the evidence already developed in this paper and from the state- 

 ments which follow below, it seems probable that the minerals at this 

 locality were not all formed at the same time nor under the same condi- 

 tions. Pyrophyllite, a hydrous silicate of aluminum, belongs to the kao- 

 linite group of minerals, the better known members of which are secondary, 

 the products of hydrous alteration of other species, t chiefly feldspars. 

 They are produced under the conditions of katamorphism. Stellate struc- 



* Genth, F. A., Corundum, Its Alterations, and Associated Minerals. Proc. Amer. 

 Phil. Soc, 1873, vol. 13, p. 382. 

 t Clarke, F. W., Bull. 491, U. S. Geol. Survey, 1911, p. 395. 



