THE ORIGIN OF FOLIATED CRYSTALLINE ROCKS 247 
some combination of prismatic and pyramidal faces, the former, 
however, being almost always present. In Fig. 8, representing 
zircons of the Butte granite, basal pinacoids may be recognized 
but this, apparently, is not a common development. It was noted 
by the writer that the cross-sections of zircons are generally oblong 
rather than square. 
2. Twinning.—T winning in microscopic grains has never been 
with certainty recognized by Thtirach. The writer’s experience 
has been similar. Geniculated twins were observed by Derby in 
the granite from Somerville, Me. 
3. Zonal banding.—Frequent. 
4. Color.—In fresh crystalline rocks almost always colorless. 
The writer has observed grains considered to be zircon which were 
slightly brownish. 
5. Optical characters.—High refringence. Brilliant interference 
colors of high order, usually red and green, less commonly yellow or 
blue. 
6. Inclusions—Numerous. Considered by Thiirach to consist 
of apatite, fluids, or gases. Derby states that xenotime also exists 
as inclusions. 
Rutile has been mistaken for zircon but can be distinguished by 
its color (yellow, yellowish or reddish brown), pleochroism, twinning 
(common), absence of inclusions, and chemical reactions. 
Xenotime was not described by Thtirach. From the observa- 
tions of Derbyt the usual crystal form appears to be octahedral. 
It may be identified by the erbium line in the spectroscope or by 
chemical means. 
The writer has not observed cassiterite In microscopic crystals. 
It is undoubtedly much more limited in its distribution than zircon 
but when present may be difficult to distinguish from the latter. 
According to Lacroix? cassiterite is more frequently twinned than 
zircon. It is also generally darker in color. Gaubert* states that 
it can readily be distinguished from zircon by color reactions with 
organic substances. 
t Mineralogical Mag., XI (1897), 304-10. 
2 A. Lacroix, Minér. de la France, III (1909), 207. 
3 Paul Gaubert, Bull. soc. fran. minér., XX XIII (1910), 326. 
