Wave forms of first arrivals at various distances from explosions in the Gulf of Maine, 1961, were analyzed by Fourier integral analysis. Frequency spectra of the wave forms in the frequency range 2.6 to 25 cps were obtained. The frequency spectra of the refraction arrivals were compared with those of the direct arrivals. Statistical tests show that there is no clear dependence of the frequency content of the arrivals upon distance for either the direct or the refraction arrival, and that the two groups of arrivals definitely belong to two separate groups. Comparison with the results of a model experiment enables an estimate of the minimum possible velocity gradient at the Mohorovičić discontinuity to be made. On the assumption of a linear velocity transition at the boundary, the minimum possible velocity gradient is found to be 4 (km/sec)/km, or the maximum possible thickness of the transition layer to be kilometers.