2o6 The American Geologist. Apiii, looi. 
and chemical analysis given in tabic A, corroborate the above 
inferences. The component minerals are essentially the same, 
but in slightly, varying proportions, as for the preceding types. 
The Stone Mountain Light Gray Granite : The Stone 
mountain light gray variety is strongly contrasted with all 
other types of granite in the state. In mineralogy it differs 
only in the reversed proportion of biotite to muscovite, and is 
a biotite-bearing muscovite granite. Biotite is a constant ac- 
companiment, but is greatly subordinated to muscovite in 
quantity. This type is uniformly light gray in color over the 
entire area, but shows some variation in texture, in places. It 
is intermediate in texture between the other types of normal 
granite on the one hand, and the coarsely crystalline granite 
matrix of the prophyritic granites on the other. As a rule, the 
Stone mountain type consists of anhedra ranging in size from 
1.5-2.5 millimetres. [Plate XX, figure 2.] The potash feld- 
spars predominate with orthoclase usually in excess of micro- 
cline. The orthoclase is commonly interwoven with a second 
feldspar in microperthitic structures. Microcline is subject to 
considerable variation in amount, at times, equaling that of 
the orthoclase in places, and sinking to a minimum in others. 
Plagioclase is only subordinate to the postash feldspars in 
amount, and occurs as stout laths with the characteristic poly- 
synthetic twinning, lamellse affording as a rule low extinction 
angles on the base. Quartz, in addition as irregular interstitial 
grains, is common in drop-like inclusions in the feldspars. 
Muscovite occurs either as individual flakes or aggregates with 
strong double refraction and often a faint yellowish tinge. 
The ray vibrating parallel to the cleavage shows appreciable 
absorption in many flakes. Biotite, as single foils Avith the 
usual pronounced color and absorption, is associated with the 
muscovite. Accessory apatite and zircon, and secondary chlo- 
rite, complete the list of microscopic minerals present. (See 
analysis I, table A.) 
Several minerals not met in thin sections of the Stone 
mountain granite are frequent microscopic accompaniments in 
the rock. The most abundant one of these is black tourma- 
line. This mineral is present, in every block of stone quar- 
ried, in the shape of aggregated small prismatic crystals usu- 
ally without terminal faces, embedded in a perfecth* white ir- 
