_a._ Conglomerate. _b._ Pebbly Sandstone, _c._ Thin-bedded Sandstone,
_d._ Shelly Sandstone, _e._ Shale. _f._ Limestone.]
They are called "stratified" because they are in themselves made up of
distinct layers, and also because they lie thus one upon another in
layers, or _strata_, just as the leaves of a book lie, or as the
bricks of a house are placed.
Throughout the greater part of Europe, of Asia, of Africa, of North
and South America, of Australia, these rocks are to be found,
stretching over hundreds of miles together, north, south, east, and
west, extending up to the tops of some of the earth's highest
mountains, reaching down deep into the earth's crust. In many parts if
you could dig straight downwards through the earth for thousands of
feet, you would come to layer after layer of these stratified rocks,
one kind below another, some layers thick, some layers thin, here a
stratum of gravel, there a stratum of sandstone, here a stratum of
coal, there a stratum of clay.
But how, when, where, did the building up of all these rock-layers
take place?
[Illustration: THE BEACH IN THE FOREGROUND IS A ROCKY SHELF, THE
REMNANT OF THE CLIFF WHICH ONCE EXTENDED OUT TO THE ISLAND.
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