hick and
heavy above the skeleton of the fish that it bore a weight of tons
there, under the water. The close-packed mud became a stiff clay. After
more thousands of years, the sea no longer came so far ashore, for the
river had built up a great delta of land out of mud. The clay in which
the fish was hidden hardened into slate. Water crept down in the loose
upper layers, dissolving out salt and other minerals, and having harder
work to soak through, the lower it went. The water left some of the
minerals it had accumulated, calcium and silica and iron, in the lower
rock beds, making them harder than they were before, and heavier and
less porous.

When the river gorge was cut through these layers of rock, the colour
and thickness of each kind were laid bare. Centuries after, perhaps
thousands of years, indeed, the quarrymen cut out the layers fit for
building stones, flags for walks and slates for roofing. In the
splitting of a flagstone, the long-buried skeleton of the fish came to
light.

Under our feet the earth lies in layers. Under the soil lie loose beds
of clay and sand and gravel, and under these loose kinds of earth are
close-packed clays, sandstones, limestones, shales, often strangely
tilted away from the horizontal line, but variously fitted, one layer to
another. Under these rocks lie the foundations of the earth--the
fire-formed rocks, like granite. The depth of this original rock is
unknown. It is the substance out of which the earth is made, we think.
All the layered rocks are made of particles of the older ones, stolen by
wind and water, and finally deposited on the borders of lakes and seas.
So our rivers are doing to-day what they have always done--they are
tearing down rocks, grinding and sifting the fragments, and letting them
fall where the current of fresh water meets a great body of water that
is still, or has currents contrary to that of the river.

Do you see a little dead fish in the water? It is on the way to become a
fossil, and the mud that sifts over it, to become a layer of slate.
Every seashore buries its dead in layers of sand and mud.

THE CRUST OF THE EARTH

It is hard to believe that our solid earth was once a ball of seething
liquid, like the red-hot iron that is poured out of the big clay cups
into the sand moulds at an iron foundry. But when a mountain like
Vesuvius sets up a mighty rumbling, and finally a mass of white-hot lava
bursts from the centre and streams down