MINERALOGY. 73 
belong to the monoclinic system, and present principally the forms of 
pl. 34, figs. 35, 52, 67. Augite, which is a silicate of lime and magnesia, 
contains often likewise alumina, protoxyde of iron, and protoxyde of 
manganese. 
7. Hornblende. 
Hornblende is widely distributed in certain rocks, which it characterizes, 
and in an imperfectly crystallized state is fibrous and somewhat radiated. 
Like the preceding, it belongs to the monoclinic system, and fig. 75 
represents its primary form. It differs from the preceding in containing 
less lime, but more magnesia and silica, being, like it, colored by the 
protoxydes of iron and manganese; it sometimes, in addition, contains 
alumina, fluor, and water. Hornblende occurs of various colors, orey, 
white, greenish, dark green to black, and also in various modifications. | 
A variety of hornblende, consisting of very long, closely compacted, and 
interlaced crystals, frequently occurs in gneiss, mica slate, and limestone. 
Fig. 13 represents thiS variety of gneiss. 
8. Staurotide. 
Alumina combined with silex forms an extensive series of minerals, and 
in this species is frequently replaced, to a greater or less extent, by other 
minerals. In staurotide, for instance, a portion of the alumina is 
constantly replaced by protoxyde of iron. It occurs imbedded in gneiss 
and mica slate; the crystals belonging to the trimetric system (pl. 34, 
figs. 47, 70). 
9. Andalusite (disilicate of alumina). 
This mineral affords fine crystallizations of the trimetric system. Fig. 12 
represents a mass of crystals of andalusite in granite. The crystals are 
right rhombic prisms. 
10. Kyanite (disilicate of alumina). 
The chemical composition of this mineral approximates it to the last ; its 
crystals, however, belong to the triclinic system (pl. 34, fig. 65). Kyanite 
is often of a vitreous lustre, on the sometimes striated surface; the 
fracture is iridescent; the color sky blue, sometimes white. It occurs 
usually in bladed crystallizations, in mica slate, gneiss, dolorite, and 
limestone ; it is rarely found amorphous. 
11. Topaz (fluo-silicate of alumina). 
Topaz is distinguished for its hardness. It exceeds quartz in this respect, 
and occurs imbedded, in very perfect crystals of the trimetric system, in 
gneiss and granite. It is found in great beauty and large quantity, in the 
topaz rock of Schneckenstein, in Saxony. The crystals from this locality 
have generally the appearance presented in pl. 35, fig. 9. Its composition 
may be set down at two thirds silicate of alumina, with fluo-silicic 
acid. 
12. Humboldtite (silicate of alumina and lime). 
This mineral is found on Mount Vesuvius. It crystallizes in shapes of 
the square system. It is very hard and scratches glass, has a yellow color 
varying to green, and melts before the blowpipe into a_ vesicular 
sub-transparent glass. Its crystalline form is shown in pl. 34, fig. 55. 
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