248 J. D. TRUEMAN 
THE FORMATION OF ZIRCON AND ITS CAPACITY FOR RESISTING 
ALTERATION 
Crystallization from a magma.—Microscopic evidence shows 
beyond any doubt that the minute zircons (they are rarely over 
o.5 mm. in length) which seem to occur in nearly every igneous 
rock have in general crystallized as an original constituent. More- 
over such methods as are in use for determining the order of crys- 
tallization of minerals indicate that zircons have usually formed 
very early in the consolidation of the magma. 
Formation in rocks secondarily through the agency of water or 
gases.—Derby* was of the opinion that all zircons in rocks have 
resulted from the crystallization of igneous melts and that they 
could not be formed in a rock secondarily. He argues: “ Unless, 
therefore, these rare chemical agents are introduced into the mass 
subject to metamorphism by the action of the so-called mineralizing 
agents (as fluorine, boron, and tin are supposed to be in the forma- 
tion of tourmaline, topaz, and cassiterite), it is difficult to conceive 
how the minerals in question can appear as newly formed ele- 
ments in a metamorphosed sedimentary. Their early crystalliza- 
tion and uniform distribution in eruptives, as well as their absence 
from schists metamorphosed by contact (in the rare cases in which 
zircon has been noticed it may be presumed to have existed in the 
original sediment) exclude the hypothesis of such an introduction.”’ 
Thiirach, on the other hand, was of the opinion that zircons 
could form from watery solutions and cited as proof the zircons 
which occur in druses in the chlorite schist of Tyrol and also the 
well-developed crystals in the sericite schist of Taunus which is 
associated with a quartzite containing well-rounded grains. There 
is just a possibility, however, that in the latter case the sericite 
schist may have been formed from material which was not as well 
sorted as the underlying quartzite and consequently more easily 
rendered schistose. In sucha case one would not expect the zircons 
to be as water-worn as in the purer variety. 
The following two cases which have recently come to the writer’s 
attention furnish additional proof that the views expressed by 
Derby must be considerably modified. In the crystalline lime- 
1 Proc. Rochester Acad. Sci., 1 (1891), 203. 
