Piedmont Plateau of Georgia. — JVatson. 221 
and at times the distinctly bounded muscovite plates penetrate 
and cut across the biotite foils at various angles. The relations 
of the two micas to each other, very strongly point to contem- 
poraneous crystallization of the two minerals. The large shreds 
of supposed primary muscovite are in marked contrast in the 
same section to the muscovite of known secondary origin asso- 
ciated with kaolin, and resulting from feldspathic alteration. 
The cleavage lines are less distinct, and the muscovite presents 
greater irregularity of outline in case of the mineral of sec- 
ondary origin. Still another portion of the muscovite in a 
part of the Georgia rocks has developed as a result of dynamo- 
metamorphism, and it is not always easy to distinguish between 
this and the same supposed primary constituent. 
The marked similarity in mineral composition of the gran- 
ites and gneisses is plain from the above descriptions. No 
mineral is found in one, that does not occur in the other; and 
those minerals most abundant in one predominate in the other. 
Their mineral composition conforms in every essential to that 
of known igneous granites occurring elsewhere. They are 
composed of minerals which most commonly make up the 
masses of normal eruptive granite. Not a single mineral 
among the list of primary and essential ones are included in 
the three rock phases, that does not characterize an igneous 
granite. The essential minerals are present furthermore, in 
the usual proportions common to such rocks. The rock types 
studied are entirely free from such minerals as staurolite, and- 
alusite, cordierite and kyanite, frequently characteristic of 
sedimentaries. Garnet is sparingly present in some of the 
granites and becomes ver}^ abundant, in places, in a part of the 
gneisses, but it is unquestionably, in these cases, a product of 
metamorphism, and is as readily produced by such action, in 
igneous rocks, as in sedimentaries. 
Hardly without exception, the thin sections of these rocks 
disclose an abundance of ovals or rounded disks of micropeg- 
matitic structures — intergrowths of feldspar and quartz — 
\vhich from their nature, in this case, are unquestionably re- 
ferred to a primary product of the magma, and therefore rep- 
resent simultaneous crystallization of the quartz and feldspar. 
Weathering: Not only are the three phases of granitic 
rock alike in the above respects, which are in accord with sim- 
