3 
VI. Hexagonal Solids. These have four axes, three of 
them lying in a plane, and intersecting each other 
at equal angles (60°), but they are perpendicular to 
the fourth or vertical axis. This system includes— 
1. The Rhombohedron. Plate B, Fig. 18. 
2. The Hexagonal Prism. Plate A, Fig. 22. 
3. The Hexagonal Double Pyramid, Plate III., 
Fig. 1; and its derived forms, such as the 
quartzoid, Plate III., Figs. 3 and 6 ; the 
hexagonal plate, Plate XII., Figs. 5, etc.; 
the twelve-sided prism, Plate A, Fig. 26; 
and the scalenohedron, or hexagonal, unequal- 
edged, double pyramid, Plate VII., Fig. 11. 
Crystals, with a vertical axis of a variable length, are 
called open forms; when the vertical axis has a deter¬ 
mined length in relation to the transverse axes, they are 
called closed, forms. The former are always prismatic , that 
is, they crystallize in four, six, eight, or twelve-sided 
prisms, the lateral planes and edges of which lie in the 
direction of the vertical axis. The closed solids are also 
called pyramidal, when the planes are inclined towards the 
vertical axis, and terminate in an angle or vertex. Thus, 
the different octahedrons, rhombohedrons, and their com¬ 
binations, are pyramidal bodies. Ail the solids of the 
tesseral system, whose axes are equal and perpendicular 
to one another, are called, on the other hand, regular or 
spherohedral ; they have the appearance of a prism, or of 
a pyramid. 
IRREGULARITIES OF CRYSTALS. 
Irregularities of crystals occur in all the systems. 
Sometimes single planes, and sometimes certain pairs of 
planes, are more or less extended at the expense of others, 
an instance of which may be observed in the druses of 
quartz, Plate III., Figs. 6 and 7, where in single crystals 
(Fig. 7) one of the pyramidal and two of the prismatic 
planes are so extended as partially to remove the others. 
The planes of combination of truncation also sometimes 
present similar appearances, so much so, indeed, that they are 
either of unequal size, or have in part quite disappeared. 
The union of half or of whole crystals is called twin 
composition. The two parts may be parallel with the 
vertical axis, as in Plate V., Fig. 2, or cross each other in 
any way, as in Plate V., Fig. 5; Plate I., Fig. 23; Plate 
XVII., Fig. 14, etc. Two or more such twins are often 
joined to each other, as in Plate VI., Fig. 4; Plate I., 
Figs. 19 and 20. 
Aggregations of crystals form either druses , Plate 
III., Figs. 5, 6, and 7—a term which implies irregular 
aggregations generally, or they are notched, arborescent, 
or dendritic forms. Plate XV., Figs. 7, 8, and 13; Plate 
XIV., Figs. 2, 5, and 10. Sometimes they pass into 
laminated tables or plates, Plate XIV., Fig. 4; into bar¬ 
like plates, Plate XIV., Fig. 1; and also into filiform or 
capillary forms, as occurs in the native metals, and in many 
ores. At other times they are found in the form of long 
needles, which are not uncommonly arranged in bundles 
(acicular crystals) , Plate XVI., Figs. 8 and 11 ; radiating 
foliated, as in Plate XVI., Fig. 7 ; and concentric radiated 
forms, as in Plate XVI., Fig. 10, also occur, as well as 
compact wedge-like pieces, Plate XX., Fig. 22, 
Pseudomorphs are those crystalline bodies -whose inter¬ 
nal constitution and elements do not correspond to the ex¬ 
ternal form. They are partly formed by alteration, as, for 
instance, crystals of pyrites which are changed into brown 
and red ironstone, by having given off their sulphur and 
taken up instead oxygen and w T ater, or the former alone. 
Another example is presented by a small crystal of anjr 
mineral which is so incrusted by, or imbedded in another, 
as to be quite invisible, or at last to have entirely 
disappeared. Instances of this kind are called pseudo¬ 
morphs by incrustation. Cases where the space, which 
has been occupied by another crystal, is so filled by a 
foreign mineral as to produce almost a copy of the former, 
present a third form of pseudomorphism, and are called 
pseudomorphs by substitution. 
NON-CRYSTALL1ZED IRREGULAR FORMS. 
Although all true minerals which exist either as simple 
elements or exhibit fixed chemical composition, occur 
crystallized, there are, nevertheless, many which present 
quite irregular, or completely shapeless masses, or such 
forms as cannot be reduced to crystals; such are the 
globular, botryoidal , reniform masses. Many of these, such 
as Carrara marble, internally present traces of crystalline 
structure, and are therefore said to be crystalline ; some, on 
the contrary, present a perfectly clear, glass, or resin-like 
fracture, as, for example, the opal, Plate IV., Figs. 19, 
20, or amber, Plate XII., Fig. 6, and these are called 
amorphous. Others again are earthy, readily part with 
colour, and are easily crumbled into dusty particles, as, for 
instance, ochre, clay, chalk, etc. 
It is clear that the purposes to which a mineral may 
be applied must, in many instances, depend upon these 
different conditions of aggregation; and all the internal 
relations, which are readily made apparent by breaking or 
tearing minerals, are comprehended under the names of 
Structure and Fracture. It is evident that those bodies 
which are derived from the vegetable kingdom, as, for 
example, many resins and coals, Plate XII., Figs. 6-13, 
have retained their original form with the corresponding 
structure, and this also is true as regards particular petri¬ 
factions. 
HARDNESS. 
The hardness of any mineral may be best tested by a 
cutting instrument, such as a knife, steel, or file, or by a 
mineral whose hardness is already determined; and as the 
expressions half-hard, hard, and very hard, are always 
indefinite, the 
newer method introduced by Mohs 
specially commended. 
His scale 
is the following: 
1 . 
Degree ; 
Hardness of Talc. 
2 . 
3 ? 
33 
Gypsum. 
3. 
33 
33 
Calcite. 
4. 
3 ? 
3 ? 
Fluor Spar.. 
5. 
33 
33 
Apatite. 
6. 
33 
V) 
Feldspar. 
7. 
33 
3 3 
Quartz. 
8 . 
33 
33 
Topaz. 
9. 
33 
33 
Sapphire. 
10 . 
3 ? 
33 
Diamond. 
