shale vs stratum

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. 

stratum

noun
  • A layer of sedimentary rock having approximately the same composition throughout. 

  • One of several parallel horizontal layers of material arranged one on top of another. 

  • Any of the regions of the atmosphere, such as the stratosphere, that occur as layers. 

  • A class of society composed of people with similar social, cultural, or economic status. 

  • A layer of vegetation, usually of similar height. 

  • The level of accuracy of a computer's clock, relative to others on the network. 

  • A layer of tissue. 

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