THE ORIGIN OF FOLIATED CRYSTALLINE ROCKS 245. 
The criterion of chemical composition, for example, has been 
especially popular, probably because it permits of mathematical 
expression and is almost free from what is called “‘the personal 
equation.” Its use, however, is based on a supposition, it being 
assumed that a rock as a whole undergoes no significant chemical 
change during the development of foliation. 
The use of zircon, on the other hand, is especially attractive 
since it deals with first-hand evidence, and while its limitations 
are not yet fully defined they can be determined with no great 
effort by laboratory research. In brief, the proposal regarding the 
use of this mineral as a criterion for the determination of the origin 
of foliated rocks is based on observations which show that zircon 
is present in nearly all igneous rocks as minute crystals and that 
it is practically absent from argillaceous sediments, having been 
concentrated during sedimentation in the arenaceous deposits. 
During this process, also, the zircons tend to assume a worn and 
rounded appearance. 
Derby' was the first to appreciate the possibilities of zircon as a 
criterion, his article outlining the suggestion appearing in 18901. 
The method does not seem, however, to have been taken up by 
many geologists, probably more through a lack of advertisement 
than from any inherent difficulty or weakness. Important use 
has been made of zircon and the somewhat similarly occurring 
minerals monazite and xenotime by Derby and his co-workers 
during active fieldwork in Brazil.’ 
On considering the possibilities of zircon as a criterion, one 
naturally thinks of such questions as the following: Is the presence 
or absence of zircon sufficiently characteristic of certain rock classes, 
such as igneous or sedimentary, so that it can be used as a criterion 
for their recognition after alteration? Is the character of the 
zircon grains in different rocks sufficiently distinctive in order that 
it may serve to identify them? Do zircon grains ever form or 
recrystallize during the development of foliation ? 
tC, A. Derby, Proc. Rochester Acad. Sci., 1 (1891), 202. 
2 Tt was from Dr. Leith, who has recently had the opportunity of observing the 
methods of the Brazilian geologists, that the writer received the suggestion to investi- 
gate this subject. 
