Heat plays an important part in the splitting of rocks and in the formation of debris.  Rocks in exposed places are greatly affected by changes in temperature, and in regions where the changes in temperature are sudden, severe, and frequent, the rocks are not able to withstand the strain of expansion and contraction, and as a result crack and split.  In the Sahara Desert much crumbling of the rock into sand has been caused by the intense heat of the day followed by the sharp frost of night.  The heat of the day causes the rocks to expand, and the cold of night causes them to contract, and these two forces constantly at work loosen the grains of the rock and force them out of place, thus producing crumbling.

[Illustration:  Fig. 6.—­Splitting and crumbling of rock caused by alternating heat and cold.]

The surface of the rock is the most exposed part, and during the day the surface, heated by the sun’s rays, expands and becomes too large for the interior, and crumbling and splitting result from the strain.  With the sudden fall of temperature in the late afternoon and night, the surface of the rock becomes greatly chilled and colder than the rock beneath; the surface rock therefore contracts and shrinks more than the underlying rock, and again crumbling results (Fig. 6).

[Illustration:  Fig. 7.—­Debris formed from crumbled rock.]

On bare mountains, the heating and cooling effects of the sun are very striking(Fig. 7); the surface of many a mountain peak is covered with cracked rock so insecure that a touch or step will dislodge the fragments and start them down the mountain slope.  The lower levels of mountains are frequently buried several feet under debris which has been formed in this way from higher peaks, and which has slowly accumulated at the lower levels.

5.  Temperature.  When an object feels hot to the touch, we say that it has a high temperature; when it feels cold to the touch, that it has a low temperature; but we are not accurate judges of heat.  Ice water seems comparatively warm after eating ice cream, and yet we know that ice water is by no means warm.  A room may seem warm to a person who has been walking in the cold air, while it may feel decidedly cold to some one who has come from a warmer room.  If the hand is cold, lukewarm water feels hot, but if the hand has been in very hot water and is then transferred to lukewarm water, the latter will seem cold.  We see that the sensation or feeling of warmth is not an accurate guide to the temperature of a substance; and yet until 1592, one hundred years after the discovery of America, people relied solely upon their sensations for the measurement of temperature.  Very hot substances cannot be touched without injury, and hence inconvenience as well as the necessity for accuracy led to the invention of the thermometer, an instrument whose operation depends upon the fact that most substances expand when heated and contract when cooled.