THE ORIGIN OF THE PHENOCRYSTS 121 



case the biotite inclusions are readily distinguished megascop- 

 ically. 



The comparative abundance of inclusions x in the pheno- 

 crysts, and the form and size of the latter, suggest a rapid 

 growth for the porphyritic crystals. The inclusions are not 

 limited to and distributed through the outer zones of the pheno- 

 crysts, indicative of different periods in crystal growth with ref- 

 erence to the groundmass constituents, but, on the contrary, 

 they are scattered through all parts of the crystal (phenocryst). 

 The inclusions are grouped, with few exceptions, without regard 

 to crystallographic lines or directions, and without uniform orien- 

 tation with reference to the host and each other. No external 

 evidence in the nature of crowding and pushing aside of the 

 adjacent groundmass microlites during the growth and expan- 

 sion of the phenocrysts has been observed, resulting in some 

 cases, as mentioned by Pirsson, 2 in the resemblance to the flow 

 structure. 



The microscope, as a rule, fails to indicate, in the rock sec- 

 tions studied, rounding or irregularity in crystal outline of the 

 original phenocryst resulting from a partial resorption or corro- 

 sion of the crystals by the magma in the Georgia areas. 



Phenocrysts of roughly idiomorphic outlines — flat, tubular- 

 and irregular-allotriomorphic forms — appear, with the former 

 predominating in most of the areas. In view of confirmatory 

 evidence, elsewhere stated in this paper, idiomorphism among 

 the phenocrysts in the Georgia rocks could in no-wise be 

 accepted as resulting from formation at greater depths and 

 under entirely different conditions from the other constituents. 

 In the absence of all other evidence it would be difficult to 

 prove that form alone was a definite criterion favoring intratel- 

 luric origin. Pirsson 3 has shown that contrast in crystal form 

 and size may very well be explained in an entirely different 

 way. 



''■Ibid., p. 80. 2 Op. cit., pp. 276, 277. 



3 Ibid., pp. 278-280 ; see also Crosby, W. O. : Amer. Geol., 1900, Vol. XXV, pp. 

 299-310. 



