Geochronology assumes that radioactive isotopes have been decaying exponentially during geologic history. Two independent yardsticks have been suggested to check this assumption: 1) fine growth lines on coral epitheca, and 2) maximum thickness deposited in a system plotted against its duration. A proportional relationship exists between the duration and the amount of sediment deposited in stratigraphic units. A check on the present time-scales can be made with a pair of perspective sections when the section-pair contains at least 2 system boundaries, represented by normal contacts.

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