00 TERMINOLOGY. 



* 74. IRREGULAR COMPOSITION. GROUPS AND GEODE 

 OF CRYSTALS. 



If several loose or imbedded crystals are merely aggre- 

 gated, so that one becomes the support of the other, 

 while there exists no general support, the assemblage is 

 termed a Groupe of Crystals; if, however, several aggre- 

 gated crystals are fixed to a common basis, so as to de- 

 rive from it a general support, the assemblage is said to be 

 a Geode of Crystals. 



The difference between these two sorts of assemblages is the 

 same as that existing between an imbedded and an implanted crys- 

 tal. Both the groupes and the geodes refer only to compound min- 

 erals, never to such as are mixed. There is sometimes a degree of 

 order observable in these groupes of crystals ; it never amounts, 

 however, to a geometrical regularity, and therefore, no regular form 

 can be said to arise from the assemblage, 



. 75. IMITATIVE SHAPES. 



The shape of a compound mineral is called an imitative 

 or particular external shape, if it bears some resemblance 

 to the shape. of another natural or artificial body. Some 

 of these forms are produced in a space not incumbered 

 with matter, and depend upon the properties peculiar to 

 the minerals themselves, without being influenced by any 

 contiguous matter ; others owe their shape to that extra- 

 neous or foreign matter with which they are surrounded. 

 The latter of these have been called extraneous imitative 

 shapes. 



The groupes and geodes are the simplest modes in which the ir- 

 regularly compound minerals appear in nature. If the individuals 

 thus connected are diminished in size, and if their number at the 

 game time increases, imitative forms are produced from the groupes 



