There _must_, therefore, always have existed, at some part or another of the earth's surface, areas where no deposition of rock was going on, and the proof of this is to be found in the well-known phenomenon of "_unconformability_." Whenever, namely, deposition of sediment is continuously going on within the limits of a single ocean, the beds which are laid down succeed one another in uninterrupted and regular sequence.

Such beds are said to be "conformable," and there are many rock-groups known where one may pass through fifteen or twenty thousand feet of strata without a break--indicating that the beds had been deposited in an area which remained continuously covered by the sea.

On the other hand, we commonly find that there is no such regular succession when we pass from one great formation to another, but that, on the contrary, the younger formation rests "unconformably," as it is called, either upon the formation immediately preceding it in point of time, or upon some still older one.

The essential physical feature of this unconformability is that the beds of the younger formation rest upon a worn and eroded surface formed by the beds of the older series (fig.

18); and a moment's consideration will show us what this indicates.