190 THOMAS L. WATSON 
The percentage ratio of the alkalies in aplites, potash and 
soda, forms a ready basis for grouping them into potash-aplites, 
soda-aplites, and aplites of equal potash and soda percentages. 
The Stone Mountain aplite, as shown in the analysis, forms a 
striking illustration of the third or last type in which the per- 
centage ratio of the potash to soda is equal. To make more 
emphatic this grouping, and for convenience of comparison, I 
have tabulated above analyses of some of the recently described 
well-known aplites from the eastern and western United States: 
Tourmaline areas.— A noteworthy feature of the Stone Moun- 
tain granite is the somewhat abundant occurrence of small areas 
of aggregated black tourmaline crystals throughout the entire 
mass of granite, so far as revealed by quarry operations. Hardly 
a block of the stone is quarried that does not show a few of 
these areas. The occurrence of the mineral is not that of a 
characterizing accessory, as has been noted in some granites, as 
at Predazzo in the Tyrol, in which the tourmaline takes the place 
of mica or amphibole, but is more after the order of segrega- 
tions in the biotite-bearing muscovite granite. Neither are the 
tourmaline aggregates sufficiently numerous and crowded together 
in the granite, nor of large enough size, to add to the color of 
the rock. While clearly visible in every case they do not in any 
measure detract from the good qualities of the stone for build- 
ing purposes, for the reasons already stated, and also because of 
the practical unalterable nature of tourmaline under normal 
atmospheric conditions. 
The tourmaline rarely occurs as isolated or single crystals 
in the granite proper, but nearly always as radiating and roughly 
parallel groups, which occupy the centers of perfectly white 
areas of quartz and feldspar, from which the two micas, musco- 
vite and biotite, have been excluded. The quartz-feldspar areas 
vary in size from a fraction to several inches in diameter, according 
to the number of grouped single tourmaline individuals occupy- 
ing it; and in shape they vary from oblong, irregularly rectangu- 
lar to complete spherical or circular outlines, with all gradations 
between. The tourmaline individuals consist of slender pris- 
