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Totals in "age" categories, two data sets, Grieve-88 pts (1982) and Norton-51 pts (1994) Sources for this data: Grieve, Richard A.F., The record of impact on Earth: implications for a major Cretaceous/Tertiary impact event, Geological Society of America Special Paper 190, 1982, pp27-8. Norton, O. Richard, Rocks from Space: Meteorites and Meteorite Hunters, Mountain Press Publishing Co., Missoula, Montana, (1994)