186  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1903.  [bull. '225 
rocks  by  basic  igneous  dikes.  Finally  there  was  a  third  period  of 
dynamic  action,  which  rendered  the  massive  dikes  schistose  in  struc- 
ture and  caused  further  mineralogic  alteration  of  the  dikes  into  forms 
unlike  the  original  eruptives.  Necessarily  the  second  and  the  last] 
periods  of  disturbance  increased  the  metamorphism  of  the  country 
rocks  and  aided  in  the  mineralization. 
