Four different kinds of experimental evidence suggest that the seismically effective compressibility of a rock is of the order of one-fifth less than its compressibility as determined by the high-pressure method. Additional, specially designed experiments to test this idea are urgently needed. As it stands, the tentative conclusion implies that the crystalline Sial of a continental block is essentially granitic down to a depth not far from thirty kilometers. Below that shell a second shell of granodiorite or quartz diorite is indicated. At the depth of about forty-five kilometers is a major discontinuity, which, interpreted on the same basis, represents the interface between the second shell and crystalline basalt, or gabbro. According to Gutenberg's hypothesis, this third shell contacts with a fourth, underlying shell of vitreous basalt, the thickness of which is not now to be readily determined from the wave-velocities in depth. So far as seismological evidence goes, that thickness may possibly approach 1,200 kilometers. A shell of either crystalline or vitreous peridotite near the earth's surface is not suggested by the wave-velocities. The master discontinuity at the depth of 2,900 kilometers seems capable of at least two different explanations.

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