mullock vs overburden

mullock

noun
  • Tailings: waste rock from which the wanted gold, minerals, opal, etc., has been extracted. 

  • Rubbish, waste matter. 

  • Nonsense, rubbish. 

  • A mess; the result of a blunder. 

  • Overburden: waste material generated while searching for minerals or while mining, such as when sinking a shaft. 

verb
  • To clear waste material out of a mine. 

overburden

noun
  • The rock and subsoil that lies above a mineral deposit such as a coal seam. 

  • A sterile stratum that lies above the stratum being investigated 

verb
  • To overload or overtax. 

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