The ages of the oldest rocks are of unusual interest for many reasons (geological, as well as biological, geochemical, nuclear-physical, and astronomical). Several radioactive methods combine to confirm fully that pegmatites from Southern Rhodesia and southeastern Manitoba are of a great age, possibly the same (±5 per cent). There are measured indications that rocks from other shield areas are of Rhodesia-Manitoba age, and confirmation would provide evidence in support of world-wide orogenic (or related) events—perhaps the beginnings of continental growth (Wilson). Two lines of evidence suggest that there are rocks of substantially greater (+300–500 × 106 years or more) age; (1) the difference in age (∼ 700–1600 × 106 years) between the most probable Rhodesia-Manitoba age and the most recent estimate of the age of the earth; and (2) indications from a small number of radioactive measurements. On the other hand, a statistical examination of the bulk of available data leads to the prediction that the chance of finding a pegmatite rock substantially older than Rhodesia-Manitoba is very slight indeed.
Concretionary structures in limestone from Southern Rhodesia, which field observation shows to be older than the dated pegmatites, suggest algal growth; in which case these fossils would be older than any yet reported and allow some 2700 × 106 or so years for organisms to have evolved to their present state of complexity.
The magnitude of the age of the oldest rocks, though somewhat younger, is similar to the most recent estimates of the ages of earth, meteorites, elements, and universe, each of which are in remarkable agreement.
