The following is the fundamental principle of the theory of terrestrial evolution which has been sometimes called the “expansion theory”:—
Masses of sediment laid down in an area of subsidence are gradually lowered to regions of the earth's crust that are at a higher temperature than that in which they were deposited. The sediment, being heated, expands, is crumpled and folded internally, and, bulging up at the surface, is reduced by denudation to the form of a mountain-chain.