252 TERMINOLOGY. . 180. 



axis of revolution by the edge produced between Pr+ oo 

 and Pr + co, which is parallel to the axis of P. 



Cleavage confirms to the full extent of its application, 

 every thing that has been said here on the subject of twin- 

 crystals. For it is possible to extract from the twin- crystals 

 compound forms of cleavage, in which those parts which 

 belong to one individual represent the real forms of cleavage 

 of the species. 



. 180. IRREGULAR COMPOSITION. GROUPE AND 

 GEODE OF CRYSTALS. 



If several loose or imbedded crystals are merely 

 aggregated (. 178.), so that the one becomes the 

 support of the other, while there exists no general 

 support ; the assemblage is termed a Groupe of 

 Crystals ; if, however, several crystals of that kind 

 are fixed to a common basis, so as to produce a ge- 

 neral support for them all, 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 crystal. 



There is sometimes a certain order observable in these 

 groupes of crystals, although this is never geometrical re- 

 gularity (. 178.); and no regular form is produced in the 

 assemblage. Both the groupes and the geodes refer only 

 to compound minerals, never to such as are mixed. 



These compositions consist of individuals of considerable 

 size, which, therefore may be very easily recognized ; they 

 assume always their regular form, as soon as they are dis- 

 engaged, or cease to touch other individuals. Upon these 

 assemblages is founded the explanation of several forms of 

 compound minerals found in nature. 



