mull over vs ruminate

mull over

verb
  • To think deeply about something; to ponder, deliberate or ruminate. 

ruminate

verb
  • To meditate or ponder over; to muse on. 

  • To meditate or reflect. 

  • To chew cud. (Said of ruminants.) Involves regurgitating partially digested food from the rumen. 

adj
  • Having a hard albumen penetrated by irregular channels filled with softer matter, as the nutmeg and the seeds of the North American papaw. 

How often have the words mull over and ruminate occurred in a corpus of books? (source: Google Ngram Viewer )