56 CRYSTALLOGRAPIIY. 
to the eye, it consists either of six crystals meeting in a point, 
or of three crystals crossing one another; and besides, there are 
numerous minute crystals regularly arranged along the rays. 
Fig. 2 represents a cross (cruciform) crystal of staurolite, which 
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is similarly compound, but made up of two intersecting erys- 
tals. Fig. 3 is a compound crystal of gypsum, and fig. 4 
one of spinel. These will be understood from the following 
figures. 

Fig. 5 is a sumple crystal of gypsum ; 
if it be bisected along a 6, and the right 
half be inverted and applied to the other, 
it will form fig. 3, which is therefore a 
twin crystal in which one half has a re- 
verse position from the other. Fig. 6 is 
a simple octahedron; if it be bisected 
along the plane ab ede, and the upper 
half, after being revolved half way 
around, be then united to the lower, it 
will have the form in fig. 4. Both of these, therefore, are 
similar twins, in which one of the two component parts 1s 
reversed in position. 
Crystals like figs. 3 and 4 have proceeded from a compound 
nucleus in which one of the two molecules was reversed; and 
those like fig. 1, from a nucleus of three (or six) molecules. 
Compound crystals of the kinds above described, thus differ 
from simple crystals in having been formed from a nucleus 
of two or more united molecules, instead of from a simple 
nucleus. 
Compound crystals are generally distinguished by thetr re-en- 
tering angles, and often also by the meeting of striz at an angle 
along a line on a surface of a crystal, the line indicating the 
plane of junction of the two crystals. 
Compound crystals are called twolings, threelings, fourlings, 
according as they consist of two, three, or four united crystals. 

