WHERRY AND ADAMS: CLASSIFICATION OP MIMETIC CRYSTALS 1 55 



The cause of mimetic phenomena may be, first, extrinsic, or 

 connected with influences outside of the crystal, leading to 

 peculiarities of habit which may constitute either apparent 

 increase or apparent decrease in symmetry. It not infrequently 

 happens that the only forms present on a crystal possessing a 

 low degree of symmetry may be those which the class represented 

 has in common with other more symmetrical classes. A good 

 example of this is quartz, which though actually trigonal and 

 trapezohedral, may show only the first order prism and the 

 corresponding plus and minus rhombohedrons. If perfectly 

 developed, the only symmetry which this combination can show 

 is holohedral-hexagonal. It is here suggested that the true 

 class of a substance showing such a relationship be indicated 

 by the prefix crypto-, meaning "hidden," and the class which is 

 apparently represented by pheno-, signifying "apparent." The 

 quartz crystals showing only hexagonal forms would then be 

 fully described by stating them to be: "crypto-trigonal-trapezo- 

 hedral but pheno-hexagonal-holohedral." Other illustrations are 

 pyrite, which is often crypto-pyritohedral but pheno-holohedral 

 cubic; apatite, which is usually crypto-pyrainidal but pheno- 

 holohedral hexagonal; and so on. 



A still more frequent type of mimetic effect, though not always 

 classed as such, is the decrease in apparent symmetry due to 

 distortion or irregularity in habit produced by external in- 

 fluences. This eff'ect is, indeed, almost universally present 

 among crystals, really perfect development being practically 

 never met with. Whenever simple descriptive terms are needed 

 for this type of relationship, the prefixes cndo-, meaning "in- 

 side," and ecto-, "outside," may be used. To cite a familiar 

 example, the metal coppej-, though well known to be funda- 

 mentally cubic, or endo-cubic, is almost always distorted, and 

 may be ecto-trigonal, ecto-rhombic, or even ecto-triclinic. 



The second class of causes of mimetic phenomena may be 

 termed intrinsic, since essential features of the internal structure 

 of the crystal are responsible. The effect is connected with the 

 approach of the angles in crystals of one system or class to those 

 of another, and develops most commonly through more or less 



