246 J. D. TRUEMAN 
Since the answers to the first two questions involve a knowledge 
of the stability of zircon the third will be considered first. Pre- 
ceding this, however, a short review will be given of the methods 
used in the identification of this mineral. 
IDENTIFICATION OF ZIRCON 
Separation from other constituents —While zircon is very widely 
distributed in rocks, it is not usually present at any time in more 
than minute quantities, indeed generally under 0.4 per cent. 
For study, accordingly, the zircons in a rock must be concentrated 
in some way. Derby* has recommended that the grains be sepa- 
rated from the ground powder by washing in a Brazilian miner’s 
pan, which, as described, should be made of thick sheet copper in 
the shape of a broad cone, with sides meeting at the apex at an 
angle of 120°. Twelve inches is suggested as a suitable diameter of 
the pan at the opening. The residues obtained, Derby states, may 
be further separated, when necessary, by means of heavy solutions 
as Thoulet’s or Klein’s or with the electromagnet. H. Thiirach,? 
in carrying out a valuable and extensive series of tests for the 
purpose of determining the distribution of zircon, anatase, brookite, 
pseudobrookite, and other minerals, employed a porcelain dish 
instead of a copper pan. The writer has examined a considerable 
number of rocks in the way recommended by Derby and found 
the method very satisfactory. It was considered advisable to 
pass the powder through a sieve before panning. The screen 
generally used was one of 60 mesh. In each case a sample of the 
concentrate was mounted in Canada balsam similarly to a rock 
section. 
Identification.—The work of Thiirach and Derby has shown that 
minute grains of zircon can usually be easily and surely distin- 
guished from all other minerals except xenotime and cassiterite 
by ordinary optical methods. The following summary of the 
characteristics of zircon, except when otherwise stated, has been 
taken from the work of Thiirach. 
1. Crystal form.—Rounded grains and well-developed crystals 
of the tetragonal system. The crystals are usually bounded by 
t Proc. Rochester Acad. Sci., I (1891), 108. 
2 Wiirzburg, Phys.-Medic. Gesellsch., XVIII (1884). 
