Art. II.] Watson, Aplitc, Pcginatitc, mid Tourmaline. 23 



structure. In some cases the tourmaline is entirely confined to 

 the feldspar individual, while in others it cuts well into the 

 quartz and feldspar grains in such way as to clearly indicate its 

 subsequent formation. In cross-section the mineral appears in 

 some cases to be irregularly rounded and granular rather than 

 bounded by sharp crystalline boundaries, but most of it is so 

 very irregular that it is best described as having an exceedingly 

 ragged outline. In a few instances the prism faces are indicated 

 under the microscope. The mode of occurrence of the tour- 

 maline and its association with the feldspar suggests beyond 

 reasonable doubt its derivation in part from the feldspar, by 

 fumarolic action. 



The tourmaline cannot be regarded as a product of contact 

 phenomena, since it is generally distributed throughout the 

 entire mass of granite, so far as quarrying operations extend — 

 not more abundant at one point than at another. I have else- 

 where shown ^ that the present granite ridge, Stone Mountain, 

 is the unreduced remnant or "core" of a once more extensive 

 mass. The evidence favoring this is that, on the north, west, 

 and south sides of the ridge, a belt of the same granite, re- 

 duced to the same general level of the surrounding Tertiary 

 Piedmont plain, skirts the ridge for a distance varying from a 

 quarter to more than a mile in width. In this reduced granite 

 zone numerous quarries have been worked yielding the same 

 beautiful light gray nearly white Stone Mountain granite. The 

 rock quarried in this zone is strikingly free from the tourmaline 

 aggregates, less than a half dozen in all having been observed. 

 The areas, then, are confined to the ridge portion of the granite 

 mass, and do not characterize the border portions of the gran- 

 ite nor of the adjoining schist and gneiss where exposed. 



Black tourmaline as isolated single crystals and aggregates 

 is rather a common associate in the Stone Mountain pegmatites, 

 and in several instances veinlets of twelve and more inches long 

 of tourmaline-felt (fine acicular tourmaline), as much as an 

 eighth of an inch in width, have been noted in the granite in 



' A Report on the Granites and Gneisses of Georgia, Gcelogical Survey of 

 Georgia. In press. 



