peruse vs scratch the surface

peruse

verb
  • To look over casually; to skim. 

  • To examine or consider with care. 

  • To read completely. 

  • To go from place to place; to wander. 

noun
  • An examination or perusal; an instance of perusing. 

scratch the surface

verb
  • To barely begin; to see or do only a fraction of what is possible. 

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