r it is the sun which
brings up the rains to nourish the rivers, it is the sun which raises
the wind which lashes the waves against the shore. But there is an
auxiliary power to keep the mill in motion, and that auxiliary power
is afforded by the tides. If then we find that by any cause the
efficiency of the tides is increased we shall find that the mill for
the manufacture of strata obtains a corresponding accession to its
capacity. Assuming the estimate of Professor Darwin, that the tide may
have had twice as great a vertical range of ebb and flow within
geological times as it has at present, we find a considerable addition
to the efficiency of the ocean in the manufacture of the ancient
stratified rocks. It must be remembered that the extent of the area
through which the tides will submerge and lay bare the country, will
often be increased more than twofold by a twofold increase of height.
A little illustration may show what I mean. Suppose a cone to be
filled with water up to a certain height, and that the quantity of
water in it be measured; now let the cone be filled until the water is
at double the depth; then the surfaces of the water in the two cases
will be in the ratio of the circles, one of which has double the
diameter of the other. The areas of the two surfaces are thus as four
to one; the volumes of the waters in the two cases will be in the
proportion of two similar solids, the ratios of their dimensions being
as two to one. Of course this means that the water in the one case
would be eight times as much as in the other. This particular
illustration will not often apply exactly to tidal phenomena, but I
may mention one place that I happen to know of, in the vicinity of
Dublin, in which the effect of the rise and fall of the tide would be
somewhat of this description. At Malahide there is a wide shallow
estuary cut off from the sea by a railway embankment, and there is a
viaduct in the embankment through which a great tidal current flows in
and out alternately. At low tide there is but little water in this
estuary, but at high tide it extends for miles inland. We may regard
this inlet with sufficient approximation to the truth as half of a
cone with a very large angle, the railway embankment of course forming
the diameter; hence it follows that if the tide was to be raised to
double its height, so large an area of additional land would be
submerged, and so vast an increase of water would be necessary for t