A unified relation among the hydrodynamic velocity Vϕ, the density ρ, the mean atomic weight m, and Poisson's ratio σ is proposed for oxides, silicates, and germanates as Vϕ = Vϕ+ ‐ 0.058(m ‐ m0) ‐ 2.62(ρ − ρ0), where Vϕ+ = −1.84 + 7.51 σ + 1.98ρ and ρ0 is the density of the crystallographically isostructural material of m = m0 = 20.2. The proposed relation can be considered as a linear form of the power law relation proposed by Shankland and Chung (1974). However, the incorporation of σ into the present relation would give a better correlation among the various parameters. It would also accommodate the velocity data of α quartz as well as those of materials undergoing various phase transformations. The value of the mean atomic weight calculated from this relation for various earth models implies that iron enrichment in the lower mantle with respect to the upper mantle is not necessary.

An empirical equation of state for high compression is derived from the assumption that K(∂P/∂K) = a + bP where K ≡ ‐V(∂P/∂V). When b = 1, it reduces to the Murnaghan's assumption. The second pressure derivative of bulk modulus is needed in the equation. Since this quantity is generally not available, a method is proposed to estimate it from other readily available data and the relation that the Grüneisen's parameter γ is a function of volume alone as As examples, NaCl, MgO, CsI, and potassium are used to test the equation. Good agreement is found for all of them compared with values in the literature.

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New chemical analyses are given for samples of hortonolite dunite from the Mooihoek Mine, Transvaal, and from the type locality at Monroe, New York; and measurements are given of wave velocities to 10 kb for the Monroe hortonolite. The olivine of the Mooihoek hortonolite has the composition Fo44Fa56, but about 10% has been altered. The olivine of the Monroe hortonolite has the composition Fo37Fa57Te6, and the velocities and density are somewhat higher than those of the Mooihoek rock. Extrapolation to fayalite suggests that the compressibility changes relatively little with iron content in the forsterite‐fayalite series.

Invariant relations derived from the general theory of elasticity are discussed in terms of their usefulness for the redundancy checks of the elastic constants and their pressure derivatives. An averaging formula is proposed to calculate the bulk modulus Ks and its pressure derivative Ks′ for a cubic crystal. Application to MgO data gives consistent values for ks′ from different sets of data measured by the same technique despite discrepancies in elastic constant data themselves.