s the country (as is well {107} seen in the case of
the Secondary rocks of England), and a corresponding section showing each
bed dipping beneath its predecessor, we are apt to imagine that beneath the
uppermost bed we should find all the others following in succession like
the coats of an onion. But this is far from being the case, and a
remarkable proof of the narrow limitation of these formations has been
recently obtained by a boring at Ware through the Chalk and Gault Clay,
which latter immediately rests on the Upper Silurian Wenlock Limestone full
of characteristic fossils, at a depth of only 800 feet. Here we have an
enormous gap, showing that none of earlier Secondary or late Palaeozoic
formations extend to this part of England, unless indeed they had been all
once elevated and entirely swept away by denudation.[37]

But if we consider how such deposits are now forming, we shall find that
the thinning out of the beds of each formation, and their restriction to
irregular bands and patches, is exactly what we should expect. The enormous
quantity of sediment continually poured into the sea by rivers, gradually
subsides to the bottom as soon as the motion of the water is checked. All
the heavier material must be deposited near the shore or in those areas
over which it is first spread by the tides or currents of the ocean; while
only the very fine mud and clay is carried out to considerable distances.
Thus all stratified deposits {108} will form most quickly near the shores,
and will thin out rapidly at greater distances, little or none being formed
in the depths of the great oceans. This important fact was demonstrated by
the specimens of sea-bottom examined during the voyage of the _Challenger_,
all the "shore deposits" being usually confined within a distance of 100 or
150 miles from the coast; while the "deep-sea deposits" are either purely
organic, being formed of the calcareous or siliceous skeletons of
globigerinae, radiolarians, and diatomaceae, or are clays formed of
undissolved portions of these, together with the disintegrated or dissolved
materials of pumice and volcanic dust, which being very light are carried
by wind or by water over the widest oceans.

From the preceding considerations we shall be better able to appreciate the
calculations as to the thickness of stratified deposits made by geologists.
Professor Ramsay has calculated that the sedimentary rocks of Britain alone
have a total _maximum_ thic