BISILICATES. 249 
differs from that mineral in lustre, specific gravity, and 
greater fusibility. 
Obs. From Uté, Sweden; also from Elba (Castor or Cas- 
lorite). 
Amphibole.—Hornblende. 
Monoclinic. JA J=124° 30’. Cleavage perfect parallel 
with /. Often in long, slender, flat rhombic i 2. 
prisms (fig. 2), breaking easily transversely; 
also often in 6-sided prisms, with oblique 
extremities. Frequently columnar, with a 
bladed structure; long fibrous, the fibres 
course or fine and often like flax, with a 
pearly or silky lustre; also Jamellar; also 
granular, either coarse or fine. 
Colors from white to black, passing 
through bluish-green, grayish-green, green, 
and be ownish-green de Aa Waal lius= 
tre vitreous, with the cleavage face inclining to pearly. 
Nearly transparent to opaque. H.=5-6. G.=2°9-3-4. 
Composition. RO; Si, as for pyroxene. KR may corre- 
spond to two or morc of the basic elements Mg, Ca, Fe, Mn, 
Na, K,, the first three being most common. Aluminum 
is very often present in amphibole, replacing a portion 
of the ate The blowpipe characters are like those of 
pyroxene. It fuses, but the fusibility varies indefinitely, 
being easiest in the black varieties. 
Diff. It is distinguished by the very ready cleavages pa- 
‘allel to a prism of 1244°, while pyroxene cleaves at nearly 
aright angle (87° 5’). 
This species, like pyroxene, has numerous varieties, dif- 
fering much in external appearance, and arising from the 
same “causes—isomorphism and crystallization. 
The following are the most important varieties :, 

I. LIGHT-COLORED VARIETIES. 
Tremolite, Grammatite. 'Tremolite comprises the white 
and grayish crystallizations which usually occur in blades 
or long crystals penetrating the gangue or aggregated into 
coarse columnar forms. Sometimes nearly “tr ansparent. 
G.=2°9. Formu'a (Ca, Mg) O; Si=Silica 57-70, magnesia 
28°85, lime 13°45=100. The name is from the foreign lo- 
ality. Tremola, 1 Switzerland. 
Actinolite. The light-green varieties. It isamagnesium- 

