HEAT AND COLD. Rocks exposed to the direct rays of the sun become
strongly heated by day and expand. After sunset they rapidly cool
and contract. When the difference in temperature between day and
night is considerable, the repeated strains of sudden expansion
and contraction at last become greater than the rocks can bear,
and they break, for the same reason that a glass cracks when
plunged into boiling water (Fig. 5).

Rocks are poor conductors of heat, and hence their surfaces may
become painfully hot under the full blaze of the sun, while the
interior remains comparatively cool. By day the surface shell
expands and tends to break loose from the mass of the stone. In
cooling in the evening the surface shell suddenly contracts on the
unyielding interior and in time is forced off in scales.

Many rocks, such as granite, are made up of grains of various
minerals which differ in color and in their capacity to absorb
heat, and which therefore contract and expand in different ratios.
In heating and cooling these grains crowd against their neighbors
and tear loose from them, so that finally the rock disintegrates
into sand.

The conditions for the destructive action of heat and cold are
most fully met in arid regions when vegetation is wanting for lack
of sufficient rain. The soil not being held together by the roots
of plants is blown away over large areas, leaving the rocks bare
to the blazing sun in a cloudless sky. The air is dry, and the
heat received by the earth by day is therefore rapidly radiated at
night into space. There is a sharp and sudden fall of temperature