chorten vs dolmen

chorten

noun
  • A Tibetan stupa. 

dolmen

noun
  • More generally, any megalithic tomb, including passage graves and wedge tombs. 

  • A prehistoric megalithic tomb consisting of a capstone supported by two or more upright stones, most having originally been covered with earth or smaller stones to form a barrow. 

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