THE ORIGIN OF FOLIATED CRYSTALLINE ROCKS 251 
istic of the zircons in sedimentary rocks. Fig. 7, indeed, shows 
that, notwithstanding the cases of secondary enlargement, the 
zircons in this rock have suffered but little alteration. 
Other tests made by the writer seem to confirm the view that 
zircon is remarkably stable under the conditions present during 
the development of foliation: Three sericite schists, for example, 
considered to have been developed from quartzite all showed no 
change in the character of the zircons during the alteration of the 
Fic. 7.—Zircon grains from Rib Hill quartzite. 4o 
rock. In two of these cases the zircons of both fresh and altered 
rocks were roundish in form while in the third (illustrated in Figs. 
to and 11) they were somewhat better developed. Two quartzose 
phases of highly altered sedimentary gneisses showed abundant and 
well-rounded zircons. In two cases of schist developed from 
igneous rocks the zircons in the schists were similar to those in the 
unaltered rocks and possessed sharp crystal outlines. 
THE SIGNIFICANCE OF THE PRESENCE OR ABSENCE OF ZIRCON 
IN A ROCK 
Distribution of zircon in unaltered rocks.—Leaving aside for the 
moment the question of the stability of zircon, one may inquire 
