highland vs mountainous

highland

noun
  • A high area; land that is higher than surrounding areas. 

mountainous

adj
  • Very difficult. 

  • Resembling a mountain, especially in size; huge; towering. 

  • Having many mountains; characterized by mountains; of the nature of a mountain; rough (terrain); rocky. 

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