Piedmont Plateau of Georgia. — Watson. 219 
AloOo and CaO, and to a less degree in the FeoO., and MgO 
over the two granite facies, are shown in the granite-gneisses ; 
while the alkalies — NajO + KjO — remain very uniform and 
constant for the three rock phases. 
It will be further observed from an examination of the table 
of individual analyses, that with one or two exceptions the 
Georgia granitic rocks are normally acid granites, showing a 
general average of approximately 70 per cent, in SiO„, while 
the percentages of iron and magnesia are somewhat below the 
general average for normal granites. 
The molecular ratios of the oxides given above show a 
tendency in the AI2O3, FcoOo and CaO to graduall}' increase as 
the SiO, decreases for the three rock phases. The gradual 
rise in ALO, with decreasing SiOs is well illustrated in the 
case of the granite-gneisses. The table of molecular propor- 
tions further shows the alkalies in sum total remain fairly con- 
stant with only slight variation. 
These statements would seem to indicate, that no absolute 
relation exists between the AUOg, SiOa and alkalies, since the 
tendency is for the AUO3 to increase as the SiOo decreases, 
while the NaoO + K„0 remain approximately the same. 
3. MINER.\L COMPOSITION. 
From the preceding descriptions, the Georgia granitic rocks 
are seen to be made up of mixtures of the essential minerals, 
quartz, feldspar and biotite, with varying amounts of mus- 
covite intimately associated with the biotite. The potash feld- 
spar varieties predominate ; and, in two cases, biotite is sub- 
ordinated to muscovite. The entire absence of hornblende in 
these rocks is a marked feature. Besides these, the usual ac- 
cessory minerals common to granite rocks in general, occur, 
and have already been mentioned. The relative proportions 
of the essential minerals in the granites, excepting the Stone 
mountain type, may be expressed thus : Feldspar including all 
varieties present > quartz > biotite > muscovite. For Stone 
mountain the proportions become : feldspar > quartz > mus- 
covite > biotite. 
There is nothing especially noteworthy about the bulk of 
the minerals present in the Georgia acid rocks, and therefor?, 
