THE microscopic glassy objects which occur in sediments deposited in the Australasian area during and shortly after the last magnetic polarity reversal are apparently tektites (see succeeding communication). The last reversal occurred 0.7 × 106 yr ago: potassium–argon dates1 indicate that the Australasian tektites were formed 0.7 × 106 yr ago. The tektites were formed and deposited at the same time as the geomagnetic field reversed, and so both phenomena could have a common cause.