THE ORIGIN OF FOLIATED CRYSTALLINE ROCKS 257) 
igneous contacts must also be kept in mind, though, as was 
stated, cases where minute zircons have been clearly introduced 
at contacts are not known. 
2. Abundant grains of well-crystallized zircon, especially when 
uniform in character and fresh in appearance indicates that the 
original rock was igneous. ‘There are, however, the possibilities of 
recrystallization of the zircons in an arenaceous sediment during 
dynamic metamorphism, introduction near igneous contacts and 
preservation of crystal form during sedimentation. 
3. Abundant grains of well-rounded zircon, especially when 
possessing a worn appearance like ground glass strongly indicates 
a sedimentary origin. Zircons, however, occur quite frequently 
as roundish grains in igneous rocks, being especially common in the 
‘more basic types. 
4. Absence of zircon grains in quartzose bands in a metamorphic 
rock indicates that such bands do not represent sedimentary layers 
but were probably deposited from solution. 
5. Similarity in character of zircon grains may be used in 
identifying the unaltered equivalent, when such exists, of the 
metamorphic rock. Derby has employed this method in Brazil 
in mapping the distribution of badly weathered rocks. 
THE USE OF MONAZITE AND XENOTIME AS CRITERIA FOR DETERMIN- 
ING THE ORIGINAL CHARACTER OF FOLIATED ROCKS 
Monazite (Ce, La, Di), PO,, has been mentioned by Derby as 
possibly having an application similar to zircon in the determination 
of the igneous or sedimentary origin of schists and gneisses. Derby’ 
has discussed various methods for the identification of monazite 
in minute grains. In addition to ordinary microscopic tests, 
microchemical reactions and examination with a hand spectroscope 
are recommended. Monazite, according to Derby, is almost uni- 
versally present in muscovite granites and their gneissic equivalents 
and frequently occurs in biotite granites. Tests so far made by him 
indicate that it is lacking in the amphibole granites and all other 
more basic rocks. As in the case of zircon, monazite becomes con- 
centrated in the arenaceous deposits during sedimentation. The 
t Am. Jour. Sci., 4th Ser., X (1900), 217-21. 
