THE ORIGIN OF FOLIATED CRYSTALLINE ROCKS 255 
rocks derived from sand showed that the zircons were generally 
more rounded than is usual in igneous rocks. 
From a consideration of the above facts, it must be acknowledged 
that the zircons in igneous rocks are sometimes roundish in char- 
acter and that in sedimentary rocks, if the original materials were 
not subjected for considerable time to the abrasive and sorting 
action of rivers, waves, and wind, perfect crystal forms may have 
been preserved. Notwithstanding these possibilities it seems safe 
Fic. 9.—Zircons from quartzite, Gowganda, Ont. X4o 
to say that the presence of good crystal outlines in zircons of a 
foliated rock indicates that the original rock was probably igneous 
while rounded forms suggest a sedimentary origin. When the 
rounding is pronounced the proof of sedimentary character is 
strong, but when it .is slightly developed it is of little signifi- 
cance. 
The luster of zircons in igneous and sedimentary rocks.—The 
luster of zircon grains is a character closely allied to their form. 
According to the writer’s observations the zircons in igneous rocks 
usually have a clear, fresh, vitreous appearance while those in 
sedimentary rocks frequently have a dull, pitted look like ground 
