Some of the conclusions in Hedberg's valuable paper on the effect of the compacting of clay rocks upon the structure of strata overlying buried hills seem to be more sweeping than the data warrant. Hedberg's average depth-porosity curve is based upon assumptions which greatly affect its accuracy. The present writer points out that the available data indicate that the void ratio of shale (the relative proportion of voids and solids) varies inversely as the depth of burial. This empirical relation is but a slight modification of a theory proposed by Sorby, it is similar to van der Waal's (gas volume) equation, and it seems to lead to acceptable geologic conclusions.

Hedberg showed that by his average curve the present writer's suggested explanation of an anticline in western Kansas was improbable. However, by the modification of Sorby's theory, this explanation appears quantitatively possible. Even granting that the two methods of computing structure are equally logical, the large discrepancy between the results demonstrates that Hedberg's method is not sufficiently accurate for use as a quantitative test. Although several of Hedberg's other conclusions seem questionable, it is desirable to emphasize the fact that his paper opens up interesting vistas upon many fields of geologic study.