Abstract
Intensity of natural remanent magnetization of potteries from Peru, Bolivia and Mexico together with igneous rocks (lavas and pyroclastic rocks) from Mexico has been measured. The specimens cover the ages between about 0 A. D. and 1400 A. D. Most of the potteries and rocks have been found to contain magnetite or maghemite with titanium which is stable against heating in air. The intensity of NRM and its demagnetization by heating have been compared with the artificial partial TRM in laboratory so that the secular variation in the geomagnetic total force at the localities could be estimated. The results seem to imply that the intensity of the geomagnetic total force in the South and Central America at about 500 A. D. was almost twice as large as that at present but that it was only 1.2 times larger than the present one at about 0 A. D. More work will be required to find whether this change is a worldwide tendency or due to a local anomaly.
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