1g2 THOMAS L. WATSON 
microcline and oligoclase individuals as partially connecting 
irregular and ragged granules closely resembles the poikilitic 
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 tourma- 
line and its association with the feldspar suggests beyond reason- 
able 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, reduced 
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 numer- 
ous 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 granite 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, 
"A Report on the Granites and Gneisses of Georgia, Geological Survey of Georgia. 
In press. 
