736 THOMAS LEONARD WATSON 
varies in the analyses from 3.09 per cent. to 4.76 per cent., and may 
equal or even appreciably exceed (Col. III) K,O. Na,O shows a 
total average for the analyses of 3.81 per cent., and K,O of 4.33 per 
cent.; a difference or excess of ©. 52 per cent; of K7O 7, Mherexact 
composition of the biotite in these granites being unknown, it is not 
possible to compute accurately the percentage of potash feldspar 
from the analyses, but most of the K,O should be calculated as 
orthoclase. 
In each analysis Na,O has been calculated as albite, and similarly 
CaO as anorthite. By adding the albite to the anorthite a soda-lime 
feldspar is obtained which ranges from Ab,An, to Ab,An, in com- 
position, corresponding to a basic oligoclase. ‘This does not com- 
pletely accord, in every case, with optical study of the thin sections, 
which frequently show the presence of microperthite, intergrowths 
of albite with the potash feldspar, and the undoubted presence of 
Na,O in the potash feldspar. 
In 13 of the 18 available analyses of South Carolina granites, the 
percentage of Na,O approximately equals or is but slightly less than 
that of K,O; in 5, Na,O exceeds K,O; and in only one is there a con- 
siderable excess of K,O over Na,O. Molecularly, Na,O exceeds K,O 
in each analysis. On the basis then of essentially equal potash and 
soda, these granities are, to follow Brogger,* quartz monzonites and 
not normal granites. 
In defining the limits of intermediate rock-types between granites 
and diorites, the range of orthoclase in quartz monzonites is stated by 
Mr. Lindgren? as follows: “In the quartz monzonites I would give 
this mineral (orthoclase) a range from 20 per cent. to 4o per cent., 
all in an assumed total of 60 per cent. feldspars. ‘The rocks contain- 
ing more than 4o per cent. orthoclase would then be classed as 
granites, there being scarcely room for another family between the 
quartz-monzonites and the granites.” 
On this basis it will be seen by reference to the table of analyses 
that if all the KO is calculated as orthoclase, the maximum range in 
1 Die Eruptionsfolge der triadischen Emptiogesteinen bet Predazzo in Sudtyrol 
(Kristiania, 1895). 
2 ‘Granodiorite and Other Intermediate Rocks,” Amer. Jour. Sci., Vol. IX 
(1900), p. 279. 
