Polarity time scale derived primarily from potassium-argon ages and magnetic polarities determined on volcanic cooling units. Black intervals depict periods when the geomagnetic field had an orientation similar to today (normal polarity) and white intervals depict periods when the field had the opposite configuration (reversed polarity). Longer periods of predominantly one polarity (Chrons) were named after prominent individuals in the early history of paleomagnetism. The shorter intervals within the Chrons (Subchrons) were named after the locality at which they were first discovered.