problem exists for counting craters on the intermediate plains unit, because those in the diameter range of 50 to 150 km commonly occur in clusters, and it is very difficult to determine which craters of a cluster are younger than the surrounding plains unit and which are older. Sources Prepared for the National Aeronautics and Space Administration by U.S. Department of the Interior, U.S. Geological Survey. Published in hardcopy as USGS Miscellaneous Investigations Series Map I–1409, as part of the Atlas of Mercury, 1:5,000,000 Geologic Series. Hardcopy is available for sale from U.S. Geological Survey, Information Services, Box 25286, Federal Center, Denver, CO 80225 References