GUIDE TO THE MIX ERA LOGIC COLLECTIONS 



39 



tal. all the dihedral angles of which slope outward. A twinned 

 octahedron is shown in fig. 152; the penetration twin cube, com- 

 mon in fluorite is shown in lig. 153 (compare also pi. a 

 scalenohedron of calcite twinned parallel to the basal plane is 



Fig. 154 Fig. 155 



shown in fig. 154, and a tetragonal twin of cassiterite in fig. 155. 

 Aggregations of crystals frequently occur grouped in parallel 

 position as shown in pi. l x . 



aS' u t -face irregu la ri t ies 

 Surface irregularities, occurring as they do on like faces of 

 some crystals, often constitute a valuable means of determining 

 the symmetry and consequently the group and system. Such 

 markings on the faces of pyrite have been noticed in a former 

 paragraph. 1 They are due in general to various causes which 

 interrupt the perfect growth of the individual, producing low 

 parallel furrows called striae or striations, angular depressions 

 or prominences and dull faces. Curved faces are sometimes 

 produced, as in the case of diamond. 



1 See p. 16. 



