time wearing into the land, whilst in the protected and upper parts
successive beach-lines might be accumulating in a sloping surface or
terrace at the foot of the cliffs which had been lately reached: hence,
supposing the whole line of escarpment to be finally uplifted above the
reach of the sea, its basal line or foot near the mouth will run at a lower
level than in the upper and protected parts of the bay; consequently this
basal line will not be horizontal. And it has already been shown that the
summit-edges of each escarpment will generally be higher near the mouth
(from the seaward sloping land being there most exposed and cut into) than
near the head of the bay; therefore the total height of the escarpments
will be greatest near the mouth; and further up the old bay or valley they
will on both sides generally thin out and die away: I have observed this
thinning out of the successive escarpment at other places besides Coquimbo;
and for a long time I was quite unable to understand its meaning. The rude
diagram in Figure 11 will perhaps render what I mean more intelligible; it
represents a bay in a district which has begun slowly rising. Before the
movement commenced, it is supposed that the waves had been enabled to eat
into the land and form cliffs, as far up, but with gradually diminishing
power, as the points AA: after the movement had commenced and gone on for a
little time, the sea is supposed still to have retained the power, at the
exposed mouth of the bay, of cutting down and into the land as it slowly
emerged; but in the upper parts of the bay it is supposed soon to have lost
this power, owing to the more protected situation and to the quantity of
detritus brought down by the river; consequently low land was there
accumulated. As this low land was formed during a slow elevatory movement,
its surface will gently slope upwards from the beach on all sides. Now, let
us imagine the bay, not to make the diagram more complicated, suddenly
converted into a valley: the basal line of the cliffs will of course be
horizontal, as far as the beach is now seen extending in the diagram; but
in the upper part of the valley, this line will be higher, the level of the