98 THOMAS L. WATSON 



crysts) in intrusive igneous rocks were of intratelluric 1 origin. 

 Indeed, as pointed out by Pirsson, no sharp distinction hereto- 

 fore has apparently been drawn in the literature, between the 

 origin of phenocrysts in intrusive and extrusive igneous rocks ; 

 indicating probably, that the porphyritically developed mineral 

 or minerals were of the same origin in the two rock divisions 

 here designated, and were formed at much greater depths than 

 the place in which they are now found. The idea that pheno- 

 crysts in intrusive igneous rocks are not always of intratelluric 

 origin, but have, in many cases, been formed in place, and are 

 therefore contemporaneous in origin, with a part at least, of the 

 other rock constituents, was advocated by Zirkel 2 in 1893; by 

 Cross 3 in 1895 '■> an< ^ more recently by Pirsson 4 and Crosby. 5 



Pirsson and Crosby are agreed in dissenting from the old 

 and long-accepted theory, that appreciable or abrupt changes 

 in the ratio of cooling and the viscosity of the magma, are 

 essential elements in the development of the porphyritic struc- 

 ture — phenocrysts and groundmass — in intrusive igneous rocks. 



A careful field study of the granitic rocks of Georgia, by the 

 writer, during the seasons of 1898, 1899 and 1900, shows a 

 number of extensive areas of coarse-grained porphyritic granites 

 occurring within the limits of the Georgia Piedmont plateau. 

 The distribution of the porphyritic granite areas is given on the 

 accompanying map. The interior gradation or passage of the por- 

 phyritic facies, peripherally, into an even-grained coarse-textured 

 non-porphyritic granite of the same mineral and chemical com- 

 position, is readily traceable in most of the granite areas. 



1 The term " intratelluric " is here used in the same sense as that given it by Pirs- 

 son in the Amer. Jour. Science, 1899, Vol. VII, p. 272, "meaning an earlier period 

 and greater depth of the magma than that in which it came to rest." 



2 Lehrbuch der Petrographie, 1893, Vol. I, p. 737 et seq. 



3 Laccolitic Mountain Groups, U. S. Geol. Surv., Fourteenth Annual Report, 

 1895, p. 231. 



4 On the Phenocrysts of Intrusive Igneous Rocks, Amer. Jour. Science, 1899, Vol. 

 VII, pp. 271-280. 



s On the Origin of Phenocrysts and the Development of the Porphyritic Texture 

 in Igneous Rocks, Amer. Geol. 1900, Vol. XXV, pp. 299-310. 



