dripstone vs stalagmite

dripstone

noun
  • Stalactites and stalagmites collectively. 

  • A protective moulding over a door or window that allows rain to drip away from the structure. 

  • A water filter in which impurities are removed by running water through a porous stone. 

stalagmite

noun
  • Coordinate term: stalactite 

  • A secondary mineral deposit of calcium carbonate or other mineral, in shapes similar to icicles, that lies on the ground of a cave. 

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