goaf vs tailings

goaf

noun
  • That part of a mine from which the mineral has been partially or wholly removed 

  • The waste left in old workings; goafing 

  • A rick or stack (of hay, etc.) when laid up inside a barn 

tailings

noun
  • The waste that remains after the minerals have been extracted from an ore by ore dressing. 

  • The feints, the last fraction of product obtained at the end of distillation. 

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