The thorium-230: uranium-234 method of dating corals and oolites has been evaluated in detail for reliability, and various criteria have been established. Reliable ages for extensive coral formations of about 6000 and 120,000 years were obtained. A hiatus in the development of coral between 6000 and 120,000 years ago on the Pactfic atoll of Eniwetok implies that conditions did not permit coral growth during this period. The record prior to 120,000 years ago is not clear, probably because of a lack of unaltered samples.

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The extent of disequilibrium between certain members of the naturally occurring radioactive heavy element families in recent and fossil marine molluscan shells has been investigated with a view to ascertaining the conditions under which these disequilibria may serve as measures of geologic age. The results obtained lead to the following conclusions. The average uranium concentration in fossil marine shells is generally higher than in modern shells from the same region. Greater than equilibrium quantities of Ra226 are present in most modern marine shells. The predominant thorium alpha activity in shells of living marine mollusks is associated with Th228, which presumably arises from decay of Ra228 in the specimen. The Th230 concentration in modern marine shells is near zero and is probably less than 1% of the equilibrium quantity. Uranium and thorium compositions of fossil marine molluscan shells collected from terrestrial environments have generally been altered by contact with ground waters. For this reason, age determination of fossil marine carbonates by uranium decay series disequilibrium measurements, although apparently sometimes applicable to fossil corals and other fossil marine carbonates that have formed and have remained in a marine environment, is not likely to give reliable results when applied to isolated fossil marine shell specimens collected from terrestrial environments.

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