Art. II. J Watson, Aplite, Pegmatite, and Toiin/ialiiie. 21 



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 analyses, 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 : 



Tourinalinc areas. — A noteworthy feature of the Stone 

 Mountain 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 gran- 

 ites, as at Predazzo in the Tyrol, in which the tourmaline takes 

 the place of mica or amphibole, but is more after the order of 

 segregations 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 building 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, ac- 

 cording to the number of grouped single tourmaline individuals 

 occupying it ; and in shape they vary from oblong, irregularly 

 rectangular to complete spherical or circular outlines, with all 

 gradations between. The tourmaline individuals consist of 

 slender prismatic forms, varying from a fraction to several milli- 



