1 1 4 THOMA S L. WA TSON 



green opaque chlorite, and to a less degree, to a faint brown 

 pleochroic epidote. Inclusions of prismatic apatite and zircon, 

 and rounded disks or ovals of micropegmatitic structures are 

 very common. 



The phenocrysts consist of the potash feldspars, orthoclase 

 and microcline, io-30 mm long and 5-io mm broad ; tabular parallel 

 to the clinopinacoid (oio). Carlsbad twinning is common. 

 Inclusions of biotite foliae equally as large as those occurring in 

 the groundmass are very abundant in the phenocrysts. In addi- 

 tion to the biotite, they contain microscopic inclusions of irregu- 

 larly bounded crystals of the interstitial quartz and feldspar. 



The Fulton county porphyritic granite area. — The Fulton county 

 porphyritic granite area, marked E-P on the accompanying 

 map, is exposed in bowlder form over a large territory six miles 

 south of Atlanta, in the extreme southern part of the county. 

 The gradation from the interior porphyritic facies, peripherally, 

 into an even-grained granite of the same color and texture, and 

 having the same mineral and chemical composition, is more 

 gradual and more strikingly shown in this than in any one of the 

 previously described areas. Near the center of the granite mass 

 the phenocrysts compose more than 50 per cent, of the entire 

 rock, while near the transition zone — change of the porphyritic 

 to the non-porphyritic texture — the phenocrysts are very spar- 

 ingly present, not more than a half dozen are shown in a yard 

 square of the rock surface. Near the center, the phenocrysts 

 are prevailingly idiomorphic in crystal outline, while the allo- 

 triomorphic type of phenocryst characterizes the transition zone 

 of the rock mass. 



The rock consists of a medium coarse-grained groundmass 

 of quartz, the potash feldspars, orthoclase and microcline, num- 

 erous laths of polysynthetically twinned plagioclase, biotite, and 

 some muscovite, in which are embedded large potash feldspar 

 phenocrysts. Accessory apatite, magnetite, and zircon ; and the 

 alteration products, muscovite, sericite, chlorite, and kaolin are 

 noted. The microscope shows the feldspars and biotite to be 

 considerably altered in some cases. 



