And, more powerful than any of these agents of
denudation, the waves and the tides are still at work along every
coast-line, eating away into the cliffs, undermining gradually and
submerging acre after acre, and making with the refuse a shingly, or a
sandy, or a muddy beach--the nucleus of a new geological formation.

Of all denuding agents, there can be no doubt that, to the land exposed
to them, the waves of the sea are by far the most powerful. Think how
they beat and tear, and drive and drag, until even the hardest rock,
like basalt, becomes honeycombed into strange galleries and
passages--Fingal's Cave, for instance--and the softer parts are crumbled
away. But the area now exposed to the teeth of the waves is not great.
The fury of a winter storm may dash them a little higher than usual, but
they cannot reach cliffs 100 feet high. They can undermine such cliffs
indeed, and then grind the fragments to powder, but their direct action
is limited. Not so limited, however, as they would be without the tides.
Consider for a moment the denudation import of the tides: how does the
existence of tidal rise and fall affect the geological problem?

The scouring action of the tidal currents themselves is not to be
despised. It is the tidal ebb and flow which keeps open channel in the
Mersey, for instance. But few places are so favourably situated as
Liverpool in this respect, and the direct scouring action of the tides
in general is not very great. Their geological import mainly consists in
this--that they raise and lower the surface waves at regular intervals,
so as to apply them to a considerable stretch of coast. The waves are a
great planing machine attacking the land, and the tides raise and lower
this planing machine, so that its denuding tooth is applied, now twenty
feet vertically above mean level, now twenty feet below.

Making all allowance for the power of winds and waves, currents, tides,
and watercourses, assisted by glacial ice and frost, it must be apparent
how slowly the work of forming the rocks is being carried on. It goes on
steadily, but so slowly that it is estimated to take 6000 years to wear
away one foot of the American continent by all the denuding causes
combined. To erode a stratum 5000 feet thick will require at this rate
thirty million years.

The age of the earth is not at all accurately known, but there are many
grounds for believing it not to be much older than some thirty million
years. That