Velocity data in Tertiary sands and shales, obtained at 62 wells in the southern San Joaquin Valley, California, are considered statistically. A mean vertical velocity gradient of 0.464 feet per second per foot is found, and evidence is presented which indicates that, on the average, 0.360 feet per second per foot, or 78% of the total vertical gradient, is due to the effect of the pressure exerted by the present over-burden. It is found that in shales the effect of overburden is smaller than this average (0.240 and 0.231 feet per second per foot for two shale formations), and in sandier formations it is greater than the average (0.428 and 0.581 feet per second per foot for two moderately sandy formations). However, the velocity vs. depth curves for sands and for shales may intersect at any depth, and it does not appear to be possible, in general, to distinguish between the two solely on the basis of the velocity at a given depth.Comparison with similar data of Weatherby and Faust for older beds and with the experimental results of Birch and Bancroft for hard sandstone and slate suggests that these gradients cannot persist very far beyond the present maximum depth of observation of about 14,000 feet.

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