197 



hornblende, where that is not present ; or in any 

 <:ase, by a foliated appearance which is often at- 

 tended by a greater or less degree of fissility. It 

 may pass into granite, or into other rocks, as for- 

 merly remarked^ and thus becomes undefinable. 



MICACEOUS SCHIST. 



In this rock, mica and quartz form the essen- 

 tial ingredients, but the presence of garnets or 

 other occasional minerals is not excluded. The 

 mica is invariably parallel, at least to the plane of 

 the bed, as in gneiss ; whether that be straight or 

 curved, or else to the planes of the contorsions, 

 where these exist; and the rock is similarly 

 foliated, and occasionally schistose. The varieties 

 of gneiss which might be confounded with it, 

 may be distinguished by the presence of felspar, 

 which is essential to that rock ; and when this 

 mineral is difficult to distinguish in the fracture, 

 it may be recognized by the white powdery ap- 

 pearance of the weathered surfaces. According 

 to this description, micaceous schist might also 

 be confounded with some varieties of schistose 

 sandstone, of the secondary class ; but the loose 



