94 T. L. WATSON WEATHERING OF GRANITIC ROCKS OF GEORGIA 



j)orph3'ritic granites, and (3) the banded or foliated granites — gneisses. 

 Laboratory stud}^ shows the three types to be nearlj^ identical in min- 

 eral and chemical composition. In the field, the porphyritic facies of 

 the granite masses are found to grade peripherally into an even-granular, 

 medium, coarse textured granite of the same mineral and chemical com- 

 position. Likewise the gneisses are elsewhere shown to be the meta- 

 morphosed equivalents of the irruptive massive granites from which 

 tliey differ only in a pronounced banded or foliated structure, secondarily 

 induced by metamorphism. For this reason the gneisses are referred to 

 as granite-gneisses. That comparisons may be effected and emi)hasis 

 given, whatever differences arising from the influence of texture on 

 weathering, as illustrated ])y the degeneration of the Georgia granitic 

 rocks, the rock-decay is separately treated under (1) granite; (2) por- 

 phyritic granite ; and (3) granite-gneiss. The several places described 

 in the following pages are shown on the accompanying map, plate G. 



Granite 

 character axd composition 



The fresh unaltered granites are massive, fine to medium grained rocks 

 in texture, light gra}'' to dark-))lue gray in color, and show an admixture 

 of fine, granular quartz, feldspars, and biotite to the unaided e3'e. The 

 principal minerals are quartz, orthoclase with microperthitic structures, 

 microcline, })lagioclase near oligoclase, brown biotite, some muscovite, 

 and a little apatite, zircon, and magnetite. 



The quartz is frequently' intergrown with the feldspar in the form of 

 rounded ovals or disks of micropegmatitic structures, clearly indicating 

 the contemporaneous cr\'stallization of the two minerals. It is some- 

 times inclosed as drop-like inclusions in mau}^ of the larger feldspar 

 crystals. The larger individuals further show numerous lines of frac- 

 ture and undulatory extinction. 



The potash feldspars usually show good cleavages ; are intergrown 

 with a second feldspar, albite, as microperthitic intergrowths, and are 

 commonly twinned according to the Carlsbad law. The presence of 

 considerable soda in the analyses corroborates the inference that the 

 intergrown feldspar with the potash varieties is albite. Microcline varies 

 in quantit3^ but lUivy equal or even exceed in a few sections the ortho- 

 clase. Plagioclase is inferior in amount to the potash feldsi)ars, and, as 

 a rule, affords low extinction angles in basal sections, indicating an acid 

 feldspar near oligoclase. This inference is corroborated by the percentage 

 of lime shown in the analyses. The biotite is deep brown in color and 

 strongl}'' pleochroic. It is present in irregular shreds and elongated 



