According to the flight instructor, the aircraft was in cruise when a sudden severe vibration went through the airframe. The pilot shut down the engine and noted that the brass abrasion strip and part of one wooden blade had separated. The pilot landed in a field, but could not stop the aircraft before it went down an embankment and into a ravine. An airframe and powerplant mechanic who recovered the aircraft reported that he saw evidence of dry rot in the remaining wooden blade section.