2i8 The American Geologist. ^p"'- i^oi- 
No.ofK20: NaaO No. of K20:.\a20 No. of K2O: Na20 
Anal. Anal. Anal. 
1 1:1.411 IX 1:1.28) ^3 XVI 1:1.641 
02 XVII 1 :0.98 
? ? XVIII 1 : 1.37 
II 1 : 1.471 _ X 1 : l.ll 
III 1 : 1.20| ? XI 1 : 1.49 
IV 1:1.4P,|> XII 1:1.14,2= XIX 1:1.50 
V 1:2.29(:^ XIII 1: 1.68Mb XX 1:1.23 
VI l:1.51|n: XIV l:1.65|t/^^^ XXI 1:3.07 
VII 1: 1.601 • XV 1:1.12) ^' XXII 1:1.42 
Vni....l : 1.291 XVIII 1 : 1.59 
be mentioned their marked uniformity or very close agree- 
ment throughout for the three rock phases. A distinguishing 
feature is their relatively high percentage of Na^O, Mrhich is 
observed to be above the average for normal granites. The 
percentages of Na,© and K2O approximate nearly equal 
amounts in a majority of the analyses, but in many the K^O 
exceeds the NaoO, Avhile in others the Na^O is in considerable 
excess over the K„0. The high NaoO in the Georgia granites 
is traceable tothree sources : (i) the presence of comparatively 
appreciable amounts of single laths of an acid plagioclase ; (2^ 
the abundance of microperthitic intergrowths of the potash 
feldspars with a second feldspar, probably albite ; and (3) in 
the potash feldspar varieties, a part of the potassium is re- 
placed by sodium. See analysis on page 209. The general 
high range in total alkalies — NagO+KoO — is a very note- 
worthy feature in the analyses. The general average for total 
alkalies is from 8.79 per cent, in the gneisses to 9.44 per cent, 
in the granites. 
For the sake of ready comparison the following percentages 
obtained from the average of 21, 10 and 12 analyses, respect- 
ively, of the normal granites, porphyritic granites and granite- 
gneisses are hereby tabulated : 
SiOo 
Normal granites 69.67 
Porphj'ritic granites 69.28 
Granite-gneisses 73.76 
No appreciable difference is noted in any one of the seven 
constituents in case of the normal granites and their equiva- 
lent porphyritic facies, but a comparison of the analyses of the 
granite-gneisses with those of the granites, discloses several 
interesting differences among the more important constituents. 
An increased percentage of about 4 per cent, in the SiOo. and 
somewhat regular corresponding decreased percentages in the 
A1203 
FesOa 
MgO 
CaO 
NaoO 
KoO 
16.63 
1.28 
0.55 
2.16 
4.73 
4.71 
16.73 
1.75 
0.72 
2.13 
4.33 
4.59 
14.52 
1.03 
0.29 
1.14 
4.16 
4.63 
