38 On the Homozomorphism of Mineral Specie? 



It must be admitted that there are no tests of homology which 

 are of invariable application. As elsewhere in science, the 

 relations of species are to be ascertained rather by the general 

 range of characters, than by the severe application of one single 

 law. But there are important aids, and their exact value should 

 be ascertained. 



1. Cleavage. — Cleavage is one of the most important means. 

 In the trimetric system, it may take place parallel, (1) to the 

 axial sections, one or all ; (2) to the lateral planes of different 

 rhombic prisms ; (3) to octahedral planes. 



a. When cleavage is parallel to one or more rhombic prisms, 

 it is generally true that, (1) the vertical axis of the prism of 

 most perfect cleavage is the proper vertical axis of the species, 

 and also that (2) these cleavage prisms for different species are 

 homologous prisms. 



Hornblende and Augite correspond to the first of the two 

 principles 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 homceo- 

 morphous, and hence there may still be an intimate relation when 

 the cleavage forms have a simple axial ratio, as 1 : 2. Diaspore 

 and Gothite exemplify the same fact ; the former has an im- 

 perfect cleavage parallel to the prism u{ oo P2 ). Staurotide 

 and Andalusite may be viewed as another example. The oc- 

 curring forms of these species have the same relation as those 

 of Hornblende and Augite, or a ratio of 1 : 2, in the longer late- 

 ral axis, and traces of cleavage correspond ; while in Topaz, a 

 third homaeomorphous species, both forms are common, and 

 indistinct cleavages are described as occurring parallel to each. 



In some cases, when there are two cleavage prisms at right 

 angles with one another, we are required by the analogies of 

 the species to take as the vertical axis that parallel to the prism 

 of least perfect cleavage ; but such examples are rare. 



b. It is common to find a prismatic and a diagonal cleavage 

 existing together. In a single natural group of species, the for- 



