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The transition of olivine to a spinel structure in the earth's mantle has been long believed to account for a discontinuity in seismic waves some 400 km below the earth's surface. However, the time may have come to look for alternate explanations for the “400-km discontinuity,” according to a paper to be presented on May 19, 1987, at the AGU Spring Meeting in Baltimore, Md. In the paper, Hideyuki Fujisawa of the Earthquake Research Institute at the University of Tokyo (Tokyo) questions whether olivine is anywhere near as abundant in the earth's mantle as has been thought for decades.