chief·ly
 /ˈʧiːfli/  adv  
  : mainly or mostly
  — used to identify the most important part of something, reason for something, etc.
   We're chiefly concerned with protecting the environment.
   His greatest works–chiefly those written between 1640 and 1685–are still read today.
   an animal that hunts chiefly by smell
   The disease occurs chiefly in children.
   The rock is composed chiefly of quartz.