THE ORIGIN OF THE PHENOCRYSTS 1 1.7 



inclusions of apatite are the most common of the primary- 

 accessories. Some zircon and a very little magnetite occur. 

 More or less chlorite, some epiclote and muscovite are present 

 as constant secondary products from the alteration of the biotite 

 and feldspars. In many of the sections, the effects of pres- 

 sure metamorphism are frequent in the nature of crushing, lines of 

 fracture, and undulatory extinction common to some of the 

 larger quartz and feldspar crystals. 



The potash feldspars are the only porphyritically developed 

 minerals. The phenocrysts vary from allotriomorphic to flat 

 tabular idiomorphic crystals in outline, in which the (001) and 

 (010) cleavages are usually well developed, and are elongated 

 in the clinopinacoidal direction. The usual habit of the simple 

 Carlsbad twins prevails. The idiomorphic type of phenocryst 

 greatly predominates over the allotriomorphic form. As a rule, 

 the phenocrysts are very conspicuous, and are readily differ- 

 entiated from the groundmass feldspars ; although in the Coweta- 

 Campbell-Fayette counties area (C-C) the phenocrysts and a 

 portion of the groundmass feldspars seemingly grade into each 

 other. The phenocrysts invariably contain inclusions of a majority 

 of the groundmass constituents, some of which are visible to the 

 unaided eye. The inclusions are distributed, as a rule, through 

 the rock without regard to any definite arrangement or orienta- 

 tion. 



CHEMICAL COMPOSITION 



The marked uniformity in the mineral composition of the 

 various porphyritic granites from the individual areas, suggests 

 similar uniformity in chemical composition. The usual amount 

 of free quartz common to this class of rocks ; the abundance of 

 potash feldspar, with somewhat increased amounts of plagio- 

 clase and proportionally small amounts of accessory minerals, 

 indicate a normal percentage of silica and lime, an increased 

 percentage of alkalies, and comparatively small amounts of iron 

 and magnesium oxides. These inferences are well shown in the 

 following analyses, made by the writer, in the laboratory of the 

 State Survey. Attention is called in the table of analyses to the 



