expunge vs shale

expunge

verb
  • To erase or strike out. 

  • To eliminate completely; to annihilate. 

  • To delete permanently (e-mail etc.) that was previously marked for deletion but still stored. 

shale

verb
  • To take off the shell or coat of. 

noun
  • A shell or husk; a cod or pod. 

  • A fine-grained sedimentary rock of a thin, laminated, and often friable, structure. 

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