56 



CRTST ALLOG-K APH Y. 



to the eye, it consists either of six crystals meeting : .n 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, vhich 



is similarly compound, but made up of two intersecting crys- 

 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 simple crystal of gypsum ; 

 if it be bisected along a b, 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 c d e, 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 is 

 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 nucleui 

 of two or more united molecules, instead of from a sinij.le 

 nucleus. 



Compound crystals _ire generally distinguished by their re-en- 

 tering angles, and often also by the meeting of striae at an angle 

 along a line on a surface of a crystal, the line indicating the 

 plane of j unction of the two crystals. 



Compound crystals are called twoliugs, threeli?igs, fourlingi, 

 according as they consist of two, three, or four united crystals. 



