As it gradually cooled, the magma solidified on the surface and the Earth became covered with a crust of primeval rocks, mainly consisting of granite and basalt. These rocks encased our globe like a rigid shell, while inside it continued to be burning hot.
When the Earth cooled still more it shrank, and the rocky layer which covered it had to contract, like an apple whose skin wrinkles as it withers.
The shell of primeval rocks wrinkled and folded under the strong lateral pressures caused by this shrinking. In some places it sank into the underlying magma: in others it rose up towards the sky.
That is how the first mountains were formed but there is no trace of them left today. Countless disturbances over the ages have completely destroyed them. The mountains we admire today are from later periods and in some cases are comparatively recent.