36 J. D. Dana on the Homeomorphism 
Hornblende and Angite correspond to the first of the two prin- 
ciples just stated, but are well known exceptions to the second: 
the cleavage prism of one has twice the breadth of that of the 
other. ‘These species, nevertheless, are closely homceomorphous, 
and hence there may still be homology when the cleavage forms 
have a simple axial relation, as 1:2. Diaspore and Gathite ex- 
emplify the same fact ; the former has an imperfect cleavage pat- 
allel to the prism 72(a P2). Staurotide and Andalusite may be 
viewed as another example. he occurring forms of these spe- 
cies have the same relation as those of hornblende and augite, or 
a ratio of 1: 2, in the longer lateral axis, and traces of cleavage 
correspond ; while in topaz, a third homeeomorphous species, both 
forms are common, and indistinct cleavages are described as o¢- 
curring parallel to each, 
the rhombohedral. 
2. T'win-composition—In compound crystals composition 
takes place in general, parallel to planes or sections of fun- 
damental value. This is well seen in monometric forms, it 
which the only planes of composition are, (1) the faces of the 
cube; (2) the faces of the regular octahedron, or planes trunca- 
ting the solid angles ; (3) the faces of the dodecahedron, or planes 
truncating the edges of a cube. It will be observed that the 
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