claystone vs shale

claystone

noun
  • Sedimentary rock composed of fine clay particles. 

  • One of the concretionary nodules in alluvial deposits. 

shale

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

  • A shell or husk; a cod or pod. 

verb
  • To take off the shell or coat of. 

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