Igneous or Sedimentary Met amorphics. 393 



-tened spheres or lenses, kidney or bean shapes, (figs. 1-2), 

 and another which by virtue of its general form and 

 "eye "-like depressions distributed over its surface may 

 be likened to that of a potato (fig. 3). Associated with 

 more or less rounded zircons are others in the same rock 

 showing smooth, unbroken, and lustrous prism faces. 

 Two explanations can be advanced for this fact: That 

 the rounded grains had their source farther away from 

 their point of sedimentation, or that the prismatic grains 

 were transported, enveloped in a protective biotite flake. 

 The latter explanation seems to be the correct one, as 

 zircon grains attached to biotite have been observed in 

 some of the sandstones, by the writer as well as by 

 others. 4 



The study of undeformed granites led to the conclusion 

 that zircon, the earliest mineral to crystallize out, may 

 undergo during consolidation of the rock a certain 

 amount of magmatic corrosion, as is evident from a more 

 or less pronounced rounding and glassy smoothness, such 

 as observed on the edges of a melting cake of ice, of its 

 edges, principally those of its pyramidal terminations. 

 Furthermore the zircons were found to show a peculiar 

 pitting, not in isolated patches, such as might have been 

 caused by the process of crushing, but all over the prism 

 faces. It was found impossible to distinguish this kind 

 of pitting from that seen on sedimentary grains and 

 caused by the chipping of the grain during water trans- 

 port. That magmatic corrosion of zircon is possible has 

 been shown by C. Doelter, 3 who exposed this mineral to 

 contact with molten basalt and produced a broad zone of 

 corrosion around the zircon grain. The zircons in unde- 

 formed granites are generally prismatic, although some 

 were found that show a distinctly ovoid form, due to a 

 combination of magmatic corrosion and the development 

 of numerous vicinal faces. The latter already have been 

 described elsewhere. 6 



In examining igneous and sedimentary gneisses the 

 fundamental assumption is often made that zircon has 



4 A. Gilligan : The petrography of the Millstone grit of Yorkshire, Quart 

 Jour. Geol. Soc, London, vol. 75, No. 300, p. 266, 1920. 



5 C. Doelter, Handbueh der Mineralchemie, Vol. Ill, Part 1, p. 142. 



6 A. Gilligan, op. cit., p. 266. 



