. THE EARTH's COVERINGS
The earth consists of a liquid core , probably composed mainly of iron, extending over half the distance from the center to the surface , with, outside it, a solid mantle a dense rock thought to be mineral olivine above the mantle is the lithosphere, or crust . The rather sharp transition from the crust to the mantle is known as the Moho . The crust is only about three miles thick under the ocean bed, but is over twenty miles thick under the continents.
The crust itself consists of two layers a lower continuous layer of basalt and a discontinuous layer of mainly granitic rocks constituting the continents which thus "float" on the heavier material beneath them.
An interesting feature of the map of the world is the general resemblance of the eastern and western shores of the north and south Atlantic oceans.