D THEIR ORDER.

In the preceding chapters of this book the attention of the student
has been directed mainly to the operations of those natural forces
which act upon the surface of the earth. Incidentally the consequences
arising from the applications of energy to the outer part of the
planet have been attended to, but the main aim has been to set forth
the work which solar energy, operating in the form of heat,
accomplishes upon the lands. We have now to consider one of the great
results of these actions, which is exhibited in the successive strata
that make up the earth's crust.

The most noteworthy effect arising from the action of the solar forces
on the earth and their co-operation with those which originate in our
sphere is found in the destruction of beds or other deposits of rock,
and the removal of the materials to the floors of water basins, where
they are again aggregated in strata, and gradually brought once more
into a stable condition within the earth. This work is accomplished by
water in its various states, the action being directly affected by
gravitation. In the form of steam, water which has been built into
rocks and volcanically expelled by tensions, due to the heat which it
has acquired at great depths below the surface, blows forth great
quantities of lava, which is contributed to the formation of strata,
either directly in the solid form or indirectly, after having been
dissolved in the sea. Acting as waves, water impelled by solar energy
transmitted to it by the winds beats against the shores, wearing away
great quantities of rock, which is dragged off to the neighbouring sea
bottoms, there to resume the bedded form. Moving ice in glaciers,
water again applying solar energy given to it by its elevation above
the sea, most effectively grinds away the elevated parts of the crust,
the _debris_ being delivered to the ocean. In the rain the same work
is done, and even in the wind the power of the sun serves to abrade
the high-lying rocks, making new strata of their fragments.

As gravity enters as an element in all the movements of divided rock,
the tendency of the waste worn from the land is to gather on to the
bottoms of basins which contain water. Rarely, and only in a small
way, this process results in the accumulation of lake deposits; the
greater part of the work is done upon the sea floor. When the beds are
formed in lake basins, they may be accumulated in either of two very
diverse condit