The data available from the regions surrounding the Pacific Ocean during the early Tertiary indicate that the poles and continents could not have been in the positions postulated by Wegener, DuToit, or Grabau, but were in approximately the same positions as at present. During the Paleocene, the 20°C. marine isotherm was north of 49° N. latitude, by middle Oligocene time it had started to shift south of that position, by middle Pliocene it had approached its present position. During the Pleistocene it oscillated both northwards and southwards.

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