THE ROCKS AND 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.