10 A POPULAR EXPOSITION OF THE 
mineral has a perfectly indefinite shape it is said to be “ massive”’ or 
“ amorphous.” 
Structure :—In the majority of minerals, a certain kind of strue- 
ture, or, in other words, the shape as well as the mode of aggregation 
of the smaller masses of which they are composed, is always observ- 
able. Structure in minerals may be either lamellar, laminar or 
foliated, prismatic, fibrous, granular, or compact. When the mineral, 
as in most varieties of calc-spar, heavy-spar, feldspar, and gypsum, 
for example, is made up of broad tabular masses producing a more 
or less stratified appearance, the structure is said to be lamellar. 
When the tabular masses (whether straight, wavv, or curved,) become 
extremely thin or leafy, as in mica more especially, the structure is 
gaid to be laminar, or foliated, or sometimes micaceous. The scaly 
structure is a variety of this, in which the lamine are of small size. 
When the component masses are much longer than broad or deep, as 
im many specimens of tourmaline, beryl, calc-spar, &c., the structure 
is said to be prismatic or columnar. When the prismatic concretions 
become very narrow, the fibrous structure originates. Fibrous min- 
erals may have either: a straight or parallel-fibrous structure, as in 
many specimens of gypsum, calc-spar, &c.; a confusedly-fibrous 
structure, as in many specimens of augite and hornblende; or a 
radiated-fibrous structure, as in the radiated varieties of iron pyrites, 
in natrolite, wavellite, &c.,—the fibres radiating from one or more 
central points. Minerals made up of small grains or granular masses 
are said to have a granular structure; ex. granular or saccharoidal 
limestone, granular gypsum, &c. Finally, when the component par- 
ticles are not apparent, the mineral is said to have a compact struc- 
ture, as in the native malleable metals, obsidian, and most varieties 
of quartz. Hard and vitreous minerals of a compact structure (ex. 
obsidian), generally show when broken, a conchoidal fracture, or a 
series of circular markings resembling the lines of growth on the 
external surface of a bivalve shell. 
Almost all minerals, especially those of a lamellar structure, 
separate more readily in certain directions than in others. This 
peculiarity is called cleavage. The fragments resulting from “ cleay- 
age’’ have often a perfectly regular or definiteform. Thus the purer 
specimens of cale-spar, no matter what their external form, break 
very readily into rhombohedrons, which measure 105°5‘ over their 
obtuse edges. Galena, the common ore of lead, yields rectangular 
