Igneous or Sedimentary Metamorphics. 395 



are of sedimentary origin. (See illustrations, figs. 5-8.) 

 All these grains had the peculiar blisterlike covering, 

 some, like the sansage-shaped grain, showing also small 

 crystal faces, previously described. It is possible that 

 the development of the latter is due to local solution and 

 recrystallization under stress ; in any case, the blisterlike 

 covering seems to have been caused by conditions under 

 which zircon was unstable. 



Figure 9 shows three zircons imbedded in, what its thin 

 section shows to be, a normal granite of undeformed 

 hypicliomorphic granular texture, from the Norcross 

 quarry, -Stony Creek, Conn. The roundness of their out- 

 lines is striking, and a comparison of these grains with 

 those shown by Trueman and Hess in their papers pre- 

 viously cited, it is believed, gives convincing proof that 

 the origin of a gneiss cannot be determined from the 

 study of its zircons in thin sections. 



Since the ultimate source of both igneous and sedi- 

 mentary zircons is the same, it was thought that the 

 rounding due to abrasion superimposed on magmatically 

 corroded zircons in sedimentary gneisses might Jead to a 

 means of distinguishing such gneisses from those of 

 purely igneous origin. But no such distinction was 

 found to be possible. Figs. 5-8 show zircons found in a 

 distinctly igneous gneiss. It will be noticed that some of 

 the grains are pearshaped and others are more or less 

 curved. 



From the foregoing the conclusion would appear justi- 

 fied that the rounding of zircons is no criterion of the 

 sedimentary origin of the metamorphosed rocks in which 

 they are enclosed, that the degree of rounding due to 

 corrasion may not even be large enough, in some of the 

 coarser sandstones, to serve as a distinguishing char- 

 acter, and that the diagnosis of a rock, so completely 

 recrystallized as to obscure the petrological evidence of 

 its origin, cannot be effected by the use of zircon as 

 criterion. 



Petrological Laboratory, Yale University, 

 New Haven, Conn. 



Am. Jour. Sci. — Fifth Series, Vol. IV. No. 23. — November, 1922 

 26 



