GEOLOGY AND SOIL 19 6. Geology and Soil. By Geology we mean the study of the rocks, and we must at the outset explain that the term rock is used by the geologist without any reference to the hardness or compactness of the material to which the name is applied ; thus he speaks of loose sand as a rock equally with a hard substance like granite. Rocks are of two kinds, (i) those laid down mostly under water, (2) those due to the action of fire. The first kind may be compared to sheets of paper one over the other. These sheets are called beds, and such beds are usually formed of sand (often containing pebbles), mud or clay, and limestone, or mixtures of these materials. They are laid down as flat or nearly flat sheets, but may afterwards be tilted as the result of movement of the earth's crust, just as you may tilt sheets of paper, folding them into arches and troughs, by compressing their ends. Again, we may find the tops of the folds so produced wasted away as the result of the wearing action of rivers, glaciers, and sea-waves upon them, as you might cut off the tops of the folds of the paper with a pair of shears. This has happened with the ancient beds forming parts of the earth's crust, and we therefore often find them tilted, with the upper parts removed. The other kinds of rocks are known as igneous rocks, which have been melted under the action of heat and become solid on cooling. When in the molten state they have been poured out at the surface as the lava of
