blocks by means of a series of Pacific earthquakes in which at the same time he attempted to differentiate from each other two kinds of surface waves not separated by Tams. He found, on scanty material however, considerably greater differences: “The velocity of the principal waves is about 21 to 26 per cent. greater under the Pacific than under the Asiatic continent. … The times of transit for P and S[1] under the Pacific with 6° focal distance are about 13 seconds and 25 seconds respectively less than under the European continent. This corresponds to a greater velocity of about 18 per cent. for S under the ocean. … The damping of the principal waves is greater under the Pacific than under Asia. … The period in the concluding phase is greater under the Pacific than under Asia. …” All these differences point to the accuracy of our assumption that the ocean floor is composed of another, and that a heavier material. It is important to note that we are dealing here essentially with surface waves, so that these data become positive proofs of the complete absence in the ocean floor of the lighter outermost crust of rock.
It seems very natural to ask whether it would not be possible to obtain any specimens of this rock directly from the oceanic floor. It will be impossible for a long time yet to bring to the surface specimens of the “country” rock by the drag-net or other means. Nevertheless, the fact that, according to Krümmel,[2] the greater portion of the specimens brought up by dredging is volcanic, deserves attention; “predomi-
- ↑ P (undae primae) and S (undae secundae) are the designations for the first and second preliminary tremors of the seismogram, which can be traced respectively to longitudinal and transverse elastic vibrations propagated through the interior of the earth.
- ↑ O. Krümmel, Handb. d. Ozeanographie, 1, pp. 193 and 197. Stuttgart, 1907.
