from the clays which have been carried down by the rivers in the shape of
silt, but which have since become hardened, and now split up easily into
thin parallel layers. The reader has no doubt often handled a piece of
hard clay when fresh from the quarry, and has remembered how that, when
he has been breaking it up, in order, perhaps, to excavate a
partially-hidden fossil, it has readily split up in thin flakes or layers
of shaly substance. This exhibits, on a small scale, the chief
peculiarity of the coal shales.

The formation of shales will now demand our attention. When a river is
carrying down with it a quantity of mud or clay, it is transported as a
fine, dusty silt, and when present in quantities, gives the muddy tint to
the water which is so noticeable. We can very well see how that silt will
be carried down in greater quantities than sand, since nearly all rivers
in some part of their course will travel through a clayey district, and
finely-divided clay, being of a very light nature, will be carried
forward whenever a river passes over such a district. And a very slight
current being sufficient to carry it in a state of suspension, it follows
that it will have little opportunity of falling to the bottom, until, by
some means or other, the current, which is the means of its conveyance,
becomes stopped or hindered considerably in its flow.

When the river enters a large body of water, such as the ocean or a lake,
in losing its individuality, it loses also the velocity of its current,
and the silt tends to sink down to the bottom. But being less heavy than
the sand, about which we have previously spoken, it does not sink all at
once, but partly with the impetus it has gained, and partly on account of
the very slight velocity which the current still retains, even after
having entered the sea, it will be carried out some distance, and will
the more gradually sink to the bottom. The deeper the water in which it