oppress vs overburden

oppress

verb
  • To make sad or gloomy. 

  • To keep down by unjust force. 

overburden

noun
  • A sterile stratum that lies above the stratum being investigated 

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

verb
  • To overload or overtax. 

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