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. It indicates, beyond the possibility of
misconception, that there was an interval between the deposition
of the older series and that of the newer series of strata; and
that during this interval the older beds were raised above the
sea-level, so as to form dry land, and were subsequently depressed
again beneath the waters, to receive upon their worn and wasted
upper surface the sediments of the later group.
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