THE ORIGIN OF FOLIATED CRYSTALLINE ROCKS 253 
Conclusions.—The facts at hand seem to show that when zircon 
is present in a rock in considerable abundance the original rock was 
probably either igneous or a sandstone and not an argillaceous rock 
or a chemical deposit. 
The absence of zircon in.a rock is not so significant. It, how- 
ever, favors the idea that the original rock was of sedimentary 
origin and suggests somewhat strongly that it was not a granitic 
rock or a sandstone. 
THE SIGNIFICANCE OF THE CHARACTER OF THE ZIRCON GRAINS 
IN FOLIATED ROCKS 
The rounding of the crystal outlines during sedimentation.—lt 
has been previously noted in a general way that the zircon grains 
in igneous rocks have well-developed crystal outlines while those 
in sedimentary rocks are more or less rounded. The facts of 
observation should, however, be stated more fully in order that the 
amount of confidence to be placed in this distinction as a criterion 
can be determined. 
Thiirach has stated that the zircons in granites and syenites 
generally show well-developed crystal forms while many roundish 
grains occur in diorite. In basalts and dolerites he observed that 
the zircons were generally roundish and frequently showed a zonal 
banding parallel to the boundaries of the grain. In sedimentary 
rocks, according to Thiirach, the zircons are generally rounded but 
some possess distinct crystal boundaries. 
While Derby has noted that the crystal forms in igneous rocks 
are, as a rule, better developed than those occurring in sands, he 
has expressed the opinion that in the former perfectly sharp-angled 
crystals are the exception rather than the rule and apparently 
characterize the amphibolitic rather than the micaceous types. 
The investigations of Mackiet on the rounding of sand grains 
indicate that zircon is more readily rounded than quartz, probably 
on account of its higher specific gravity. Four sands discussed 
by this writer showed a predominance of rounded forms in each 
case. 
tWm. Mackie, Trans. Edinburgh Geol. Soc., VII (1897), 298-311. 
