tal sea, that stretched from the crest of
one great mountain system across to the other, and north and south from
the Laurentian Hills to the Gulf of Mexico. The great weight of the
accumulating layers of rock materials on one side, and the wasted land
surfaces on the other, made the sea border a line of greatest weakness
in the crust of the earth. The shrinking of the globe underneath caused
the break; mashing and folding followed, throwing the ridge above
sea-level, and making dry land out of rock waste which had been
accumulating, perhaps for millions of years, under the sea. The
wrinkling of the earth's crust was the result of crushing forces which
produced tremendous heat.

Streams of lava sprang out through the fissures and poured streams of
melted rock down the sides of the fold, quite burying, in many places,
the layers of limestone, sandstone, and clay. Between the strata of
water-formed rocks there were often created chimney-like openings, into
which molten rock from below was forced, forming, when cool, veins and
dikes of rock material, specimens of the substance of the earth's
interior.

Tremendous pressure and heat, acting upon stratified rocks saturated
with water transform them into very different kinds of rock. Limestone,
subjected to these forces, is changed into marble. Clays are transformed
into slates. Sandstone is changed into quartzite, the sand grains being
melted so as to become no longer visible to the naked eye. The
anthracite coal of the Pennsylvania mountains is the result of heat and
pressure acting upon soft coal. Associated with these beds of hard coal
are beds of black lead, or graphite, the substance used in making "lead"
pencils. We believe that the same forces that operated to transform clay
rocks into slate, and limestone into marble, transformed soft coal into
hard, and hard coal into graphite, in the days when the earth was young.

The word _sedimentary_ is applied to rocks which were originally laid
down under water, as sediment, brought by running water, or by wind, or
by the decay of organic substances. _Stratified_ rocks are those which
are arranged in layers. Sedimentary rocks will fall into this class.
_Aqueous_ rocks are those which are formed under water. Most of the
stratified and sedimentary rocks, but not all, may be included under
this term. Rocks that are made out of fragments of other rocks torn down
by the agencies of erosion are called _fragmental_. Wind, water, an