TABLE VL 
the Hexagonal System. 
tion positive. 
1 
[ 
Occtirrence. 
Remarks. 
Structure. 
Often 
ored 
col 
by 
iFe^ O3 in 
crevices, 
or render- 
ed con- 
fused by 
inclusions. 
Quartz in 
eruptives 
often cor 
roded. Co- 
alesces 
with or- 
thoclase to 
form mi- 
cropegma- 
tite fre- 
quently in- 
granites. 
In porphy-| 
ritic erup 
tives also 
sphaero- 
litic. 
Association. 
With ortho 
clase (and san 
idine) more 
rarely with 
plagioclase, 
biotite, horn- 
blende, and 
augite. Never 
occurs in au 
gitic oliv- 
ine-bearing 
rocks or in ne- 
pheline-or 
leucite bear 
ing rocks as a 
primary con- 
stituent. With 
muscovite and 
biotite. 
Usually in 
aggre- 
gates of 
minute 
thin hex 
agonal or 
irregular 
superim- 
posed 
plates. 
Often in 
the vicini- 
ty of the 
feldspar or 
in large 
masses in 
the mag- 
ma. 
With quartz, 
sanidine, pla- 
gioclase, au- 
gite, biotite, 
and horn- 
blende. Sec- 
ondary with 
opal and chal- 
cedony. 
Inclusions. ' Alterations. 
Poor in min 
eral inclusions 
Apatite- col- 
umns. In 
the fragment 
al states and 
granites rich 
in fluid inclu- 
sions and long 
brownish or 
blackish curv- 
ed needles. 
In eruptives 
rich in glass 
inclusions and 
gas pores. 
There are 
none^although 
changes due 
to molten 
magma are 
not rare in 
quartz from 
eruptive rocks 
See corrosion 
and seconda- 
ry glass inclu- 
sions. 
Fluid inclu 
sions. 
constituent. 
(a) In erupt 
ive rocks as 
component of 
the first order, 
as macroscop- 
lar, often de 
ceptively like 
sanidine, but 
easily distin- 
guished from 
isotropic sec- 
ic fragments j tions of it. 
in grains and|Distinguished 
crystals in 
granite with 
fluid cavities 
and in quartz 
-porphyry 
from nephe- 
line and apa- 
tite by insolu- 
bility ; from 
corundum by 
Occurrence. 
Primary com 
ponent and 
secondary in 
rhyolites, tra- 
chytes, horn- 
blende-and 
augite-ander- 
ites. In gen- 
eral, moie fre 
quent in later, 
acid erupt- 
ives and more 
rare in older 
basic erupt- 
ives. Second- 
ary in cavi- 
ties of these 
rocks and, in 
that case, usu- 
ally in large 
plates. 
and trachytes character of 
with glassy jdouble refrac- 
inclusion, anddion, from cal- 
also as acces-jcite by cleav- 
sory in manyjage and twin- 
other erupt- ming. 
ives. As con-' 
stituent of the! 
second order! 
in the magma 
of these rocks ! 
(bj In nearly! 
all^ crystallinei Microscopic 
itridymite may 
slates. 
II. Secondary 
product by 
alteration of 
silicates ; in 
diabase in 
granular ag- 
gregates, and 
in veins of 
many rocks. 
III. In water 
worn grains 
in many clas- 
tic rocks. 
IV. Forming 
a simple rock, 
as quartzite 
etc. 
be recognized 
by the form of 
its aggergates, 
while for the 
larger masses 
the optical 
characters 
serve to iden- 
tifv it. 
