I I S THOMAS L. WATSON 



prevailingly high percentage of soda in all of the rocks described 

 above. In nearly half of the analyses, the soda is slightly in 

 excess of the potash, and in the remaining ones it nearly equals 

 or is but slightly less than the potash. This high range in soda 

 is traceable, first, to the prevalence of microperthitic inter- 

 growths of albite with the potash feldspars ; next, to the amount 

 of soda-lime feldspar present in the rocks; and, lastly, to the 

 potash being replaced, in part, by soda in the straight potash 

 feldspar molecule (analyses IVa and XIa). 



GENETIC RELATIONSHIP OF PHENOCRYST TO GROUNDMASS 



Evidences of i?itratelhiric origin. — The evidences upon which 

 the formation of phenocrysts at great depths and under condi- 

 tions different from those of the groundmass constituents in 

 igneous, intrusive rocks, rest have been adequately discussed 

 by Pirsson 1 and are shown to be : (i) Contrast in size and crys- 

 tal form of phenocryst to the groundmass constituents. (2) 

 The fluidal arrangement, common in many cases, to the por- 

 phyritic minerals. (3) Irregularity of form, traceable to 

 corrosion or resorption of the crystal (phenocryst) by the 

 magma. 2 



These will be discussed in relation to the phenocrysts of the 

 Georgia porphyritic granites under the next heading. 



Evide?ices of contemporaneous origin. — The evidences favoring 

 contemporaneous origin of the phenocrysts in the Georgia 

 porphyritic granites may, for convenience, be discussed under 



(1) megascooic proofs: geologic or field observations; and 



(2) microscopic proofs : study of the thin sections of the 

 rocks. 



Both the macro- and microscopic characteristics of the 

 Georgia porphyritic granites have been described in considerable 

 detail under the individual areas above. Hence it is only neces- 

 sary here to summarize and classify the facts pointing to a con- 

 temporaneous origin for the phenocrysts. 



1 Amer. Journ. Sci., 1899, Vol. VII, p. 278. 



2 Iddings, J. P.: Bulletin, Phil. Soc. of Washington, 1889, Vol. XI, p. 77. 



