Mineral of Rocks. 
55 
granites, the following table may prove of use: — 
MICA 
is usually in thin lustrous 
leaves or plates ; 
is mostly transparent ; 
is elastic and flexible ; 
the thin leaves can be split 
up into thinner leaves ; 
Muscovite is silvery-white 
mica ; 
Biotite is a dark green or 
black mica ; 
the plates can be very easily 
scratched by a knife ; 
some decomposing micas in 
river sands are golden- 
yellow. 
HORNBLENDE 
does not occur in thin leaves 
and plates ; 
is not elastic or flexible ; 
is usually found with crystal- 
line faces in granites and 
diorites ; 
is scratched with difficulty by 
a knife; 
is usually greenish-black, 
black or brown ; 
not transparent or trans- 
lucent. 
6. OLIVINE . — Olivine is also a constituent of Basalt, 
and may oftentimes be noted in that rock with 
the unaided eye, as rather glassy-looking crystals 
of a very dark olive-green, sometimes almost 
black. When Olivine is transparent, it is used 
as a gem stone, and is then known as Chrysolite. 
7. CALCITE. — Calcite is a most abundant mineral, and 
the only common mineral that is rapidly attacked 
even by cold and weak acids. When acids act 
upon calcite, carbon-dioxide escapes with effer- 
vescence. When calcite is pure it is known as 
Iceland Spar, and is then transparent. 
