252 J.D. TRUEMAN 
regarding the distribution of zircon in unaltered rocks for this, 
indeed, must form the basis for the application of zircon as a 
criterion. 
Thiirach stated that he had recognized zircons in every igneous 
rock tested, including basalts and dolerites. Huis observations 
indicate, however, that they are most abundant in the acidic types 
such as granite and syenite. Derby, also, has expressed the opinion 
that zircons are almost universally present in eruptives. Zircons 
were recognized by the writer in every granitic rock examined but 
their abundance was found to vary greatly with specimens from 
different localities. They appeared to be much less numerous in 
basalts and other basic rocks than in the acidic varieties, none, 
for example, being detected in a test on material from a specimen 
of diabase from Gowganda, Ont. 
Zircon appears to be present in varying amounts in practically 
every arenaceous rock. ‘Thiirach reported its presence in every 
sandstone examined and the writer’s observations are similar, 
eight quartzites all showing zircon and generally abundant. 
Shales, however, appear to be comparatively free from zircon 
though Thiirach’s thorough tests revealed its presence in nearly 
every case examined. Derby says that zircon is almost absent 
from argillaceous deposits and they were only observed occasionally 
in such rocks by the writer. 
Distribution of zircons in metamorphic rocks.—Thiirach found 
zircon to be generally abundant in feldspar-rich gneisses (presum- 
ably largely of igneous origin) and less common in, but seldom 
absent from, mica gneisses (probably mostly of sedimentary origin). 
Derby’s observations show that schists free from quartz such as 
amphibolite and amphibole schist frequently show abundant zircons 
but that micaceous schists contain only comparatively few grains 
of that mineral. The writer’s observations, as was mentioned 
before, indicate that zircons undergo but little change during the 
development of foliation in a rock. 
On the whole, then, it appears that the examination of meta- 
morphic rocks does not reveal any differences in distribution of 
zircon than might be expected from the preservation of original 
grains. 
