the picture of the distribution of the magnetism of the earth. Nevertheless, up to the present there has been no success in deriving, by calculation from the observation of the earth’s magnetism, the difference between continents and oceans, apparently because it is overlain by another, much greater, field of disturbance of yet unknown origin. The latter shows no relation to the distribution of the continents, and also cannot do so, as is evident from its great alterations expressed in the secular variation. But nevertheless, the results of the study of the earth’s magnetism do not at all vitiate, according to the view of such specialists as A. Schmidt, who will not yet acknowledge without reservations the convincing character of Wilde’s experiment, the assumption that the floor of the oceans consists of more ferruginous rock. Since it is generally assumed that the iron content increases with depth in the silicate mantle of the earth, and further, that the interior of the earth is mainly composed of iron, this signifies that we are dealing with a deeper layer. Now, as a rule, magnetism disappears at red heat, a temperature which would be reached, on the basis of the usual geothermal gradient,[1] at a depth of about 15 to 20 km. The strong magnetism of the floor of the ocean must therefore occur in the uppermost layers, which appears to agree quite well with our assumption that there the weaker magnetic substances are quite absent.
Our assumption is also upheld by the study of earthquakes. E. Tams[2] has compared the velocity
- ↑ According to J. Friedlaender, Beitr. z. Geophys., 11, Kl. Mitt., pp. 85–94, 1912, the thermal conductivity of the deep igneous rocks which we are considering is even smaller (geothermal gradient of lava 17 m.), so that the thickness of the magnetic layer would only amount from 8 to 9 km.
- ↑ E. Tams, “Über die Fortpflanzungsgeschwindigkeit der seismischen Oberflächenwellen längs kontinentaler und ozeanischer
