ad
brought down from the higher lands. When this is the case, the sand borne
in suspension is the first to be deposited, and this accumulates in banks
near the entrance of the river into the sea. We will suppose, for
illustration, that a small river has become charged with a supply of
sand. As it gradually approaches the sea, and the current loses its
force, the sand is the more sluggishly carried along, until finally it
falls to the bottom, and forms a layer of sand there. This layer
increases in thickness until it causes the depth of water above it to
become comparatively shallow. On the shallowing process taking place, the
current will still have a certain, though slighter, hold on the sand in
suspension, and will transport it yet a little further seaward, when it
will be thrown down, at the edge of the bank or layer already formed,
thus tending to extend the bank, and to shallow a wider space of
river-bed.

As a result of this action, strata would be formed, shewing
stratification diagonally as well as horizontally, represented in section
as a number of banks which had seemingly been thrown down one above the
other, ending in thin wedge-shaped terminations where the particular
supply of sediment to which each owed its formation had failed.

The masses of sandstone which are found in the carboniferous formation,
exhibit in a large degree these wedge-shaped strata, and we have
therefore a clue at once, both as to their propinquity to sea and land,
and also as to the manner in which they were formed.

[Illustration: FIG. 19.--_Productus_. Coal-measures.]

There is one thing more, too, about them. Just as, in the case we were
considering, we could observe that the wedge-shaped strata always pointed
away from the source of the material which formed them, so we can
similarly judge that in the carboniferous strata the same deduction holds
good, that the diagonally-pointing strata were formed in the same way,
and that their thinning out was simply owing to temporary failure of
sediment, made good, however, by a further deposition of strata when the
next supply was borne down.

It is scarcely likely, however, that sand in a pure state was always
carried down by the currents to the sea. Sometimes there would be some
silt mixed with it. Just as in many parts large masses of almost pure
sandstone have been formed, so in other places shales, or, as they are
popularly known by miners, "bind," have been formed. Shales are for