Approximately 850 m.y. ago, the tectonics of North America changed from a pattern of scattered, locally deep epicratonic troughs to a pattern of encircling marginal miogeoclines. This change can be interpreted as indicating the start of rifting that extended almost continuously around the North American craton. This concept seems to require that North America was once an interior piece of a much larger continent and that the pieces of this fragmented continent drifted away to form new continents. A test of this hypothesis requires an analysis of the world-wide distribution of late Precambrian continents, a subject of current controversy and speculation.