Piedmont Plateau of Georgia. — Watson. 215 
Basic inclusions : Another form of evidence strongly 
favoring the irruptive nature of the granitic rocks is that of 
inclusions darker in color and considerably more basic in char- 
acter than the parent rock, met in many of the masses. It has 
not been possible to distinguish in every instance between in- 
clusions of the inclosing rock and that of darker material — 
schlieren — ba.sic segregations of the granite magma. In the 
majority of cases, the darker areas plainly represent basic seg- 
regations, which form so common a feature of slowly solidify- 
ing granite magmas. They are especially abundant in many 
of the quarries ; are darker in color than the parent granite and 
consist chiefly of biotite with subordinate quartz, orthoclase, 
plagioclase and apatite, with no evidence of foliation. They are 
rounded and irregular in outline and, in case of the gneisses, 
form lenticular patches and bands in extreme cases. As a rule, 
they are finer-grained in texture than the inclosing rock, but in 
some cases they are coarser crystalline. Inclusions of this 
character are regarded by many writers* as strong evidence of 
the eruptive origin of the rocks in which they are found. 
Pegmatitic Dikes : Closely associated pegmatitic dikes 
of varying width and granitic composition are numerous 
throughout the Plateau-crystalline complex, and are unques- 
tionably genetically connected, in many cases, with the granite- 
masses. They vary from those of granitic composition to 
pure quartz. Under this heading two kinds of material are 
met. Some are true pegmatitic intrusions, while others are 
true veins of segregation. Both are common to the porphyritic 
and non-porphyritic granite facies and to the gneisses, and are 
equally characteristic of each. They are characterized by the 
usual coarse-grained structure and consist almost exclusively 
of lustrous, cleavable pink and white-tinted feldspar with some 
quartz, and a much smaller proportion of biotite, and occasional 
muscovite. [Plate XXIIL] In many cases they are very exten- 
sive and deep-seated, while in others they are very limited in 
extent and are entirely surrounded by the granitic rock. They 
are very irregular in outline, conforming to no vmiform orien- 
tation, but cut the rock in numerous directions and at various 
* ROSENBUSCH, Massigb Gesteine, p. 62. 
Williams, G. H.. l.'ith Annual Report, U. S. G. S., p. 662. 
Kemp, J. F., Bull. (ieol. Soc. Amer.. 1899, vol. x. pp. 871:572. 
Westgate, L. G.,Jouni. ofGeul., I.s99, vol. vii, p. G4-3. 
