P. Armstrong — Zircon as Criterion, etc. 391 



Aet. XXXI Y. — Zircon as Criterion of Igneous or Sedi- 

 mentary Met amorphics; by P. Akmstrong. 



The complete recrystallization of some igneous and 

 sedimentary rocks with consequent total loss of their dis- 

 tinguishing characters has led petrographers to search 

 for some definite criterion by which, in the absence of con- 

 clusive field evidence, the genetic origin of such recrystal- 

 lized rocks can be accurately determined. Various means 

 to this end have been proposed, amongst them the use of 

 zircon. 1 This method rests on the assumption that 

 zircon, like grains of sand, may undergo rounding 

 during water transportation ; those zircons contained in 

 igneous rocks always show sharp crystal boundaries. A 

 second postulate on which this method rests is that the 

 extreme stability of zircon enables this mineral to resist 

 physical and chemical changes even under the greatest of 

 metamorphic forces. 



Doubts as to the reliability of this method, which has 

 been endorsed or employed by various geologists, 2 were 

 expressed orally to Professor A. Knopf by several petrog- 

 raphers and led to its critical investigation by the writer. 

 The conclusions arrived at are briefly set forth below. 



The rocks to be investigated were pounded, not ground, 

 in a mortar to pass an eighty-mesh sieve, panned down 

 to the heaviest constituents, and dried. Under a binoc- 

 ular microscope, equipped with a 24mm. objective, a 

 number of zircon grains, up to twenty in some trials, 

 were isolated and carefully mounted on glass fibers. 

 Each grain was then placed under a No. 4 objective of a 

 petrographical microscope and by means of the glass 

 fiber, resting on the stage, rotated so as to give a complete 

 view of the grain from all sides. In the latter operation 

 reflected light was used. 



A preliminary study of four unmetamorphosed sand- 

 stones showed that the degree to which zircons are 

 rounded diminishes with increasing coarseness of the 



1 J. D. Trueman : The value of certain criteria for the determination of 

 the origin of foliated crystalline rocks, Journal of Geology, vol, 20, No. 3, 

 1912. 



2 A. N. Winchell : The Dillon quadrangle, Montana, U. S. G. S., Bull. 574, 

 p. 129, 1914. Leith & Mead, Metamorphic Geology, 1915, p. 225 (H. Holt 

 &Co.). 



