The completeness of the geologic record obviously diminishes with the passage of time, not simply because younger rocks come to bury the older, but also because the younger have been largely derived by the cannibalizing of the older. This fact has been all too often ignored in generalizations about geologic history and the record has been treated as though all parts of the column are susceptible of equally fine discrimination. Many generalizations that burden the geologic literature—that orogeny has been speeding up with time, that erosion rates show a secular increase, and many other deductions as to trends in geologic processes—lose much, if not all, of their credibility when account is taken of this fact.

Accurate data are not available to permit quantitative evaluation of the loss in the record with increasing geologic age. It seems highly unlikely, too, that the rate of loss has been uniform through time. But map data provide a crude approach to such an evaluation and plainly show the prevailing tendency. As with optical perspective, it is clear that a feature distant in time must be larger than one close at hand if it is to appear as distinctly. Perhaps it is more accurate to say that our chances of reading the geologic record in similar detail diminish with the passage of time.

In this study I have measured on the geologic maps of North and South America the areas occupied by rocks of the several categories distinguished on them and have compared the areas of exposure of each category with the time span during which it was formed. The results are presented in tables and graphs. These data give a rough measure of the loss of detail of the record and raise some questions as to apparent secular variation in such processes as volcanism and plutonism.

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