of the Trimetric System. 39 



mer may become obsolete, while the latter is highly developed, 

 or the reverse ; and therefore the presence or absence of a diago- 

 nal or basal cleavage is no test of identity. The anhydrous sul- 

 phates are a prominent example. In Celestine and Heavy Spar 

 a basal and prismatic cleavage exist, and the two diagonal 

 cleavages are imperfect ; while in Anhydrite., of the same group, 

 the basal and diagonal are highly perfect, and no prismatic 

 cleavage has been delected. In rhombohedral forms, a basal 

 cleavage often occurs along with a rhombohedral, and in spe- 

 cies actually homoeomorphous, it may become the only cleav- 

 age, or be wholly obsolete. It is, however, often true, that a 

 particular direction of cleavage characterizes a group of species. 

 In the Heulandite group there is a perfect clinodiagonal cleav- 

 age ; the Feldspars have a basal and clinodiagonal ; the species 

 of the Calcite series have a perfect rhombohedral cleavage, and 

 no distinct basal, while the Corundum series have generally a 

 basal cleavage, more distinct than the rhombohedral. 



2. Twin- composition. — In compound crystals composition takes 

 place in general, parallel to planes or sections of fundamental 

 value. This is well seen in monometric forms, in which the 

 only planes of composition are, (1) the faces of the cube ; (2) the 

 faces of the regular octahedron, or planes truncating the solid 

 angles ; (3) the faces of the dodecahedron, or planes truncating 

 the edges of a cube. It will be observed that the composition 

 is either at the extremities of the axes (1), or at points exactly 

 intermediate between three axes (2), or between every two (3). 

 This narrow limit to the possible directions of twin-composition 

 gives importance to its indications, and therefore similarity in 

 modes of composition suggests identical or homologous relations be- 

 tween the planes of composition in different species, and vice versa. 

 Thus when we observe different species, as Aragonite, Cerusite, 

 etc., affording stellate twins and hexagonal forms by composi- 

 tion, parallel to the faces of a prism nearly 120° in angle, we infer 

 that the prisms are homologous ; and when similar prisms occur 

 in Chrysoberyl or Copper Glance, we conclude that the prism 



