In moist and equable climates these temperature changes are of very subordinate importance as a destructive agent and act chiefly in giving an easier passage to percolating waters. In arid regions, on the other hand, especially on high mountains and plateaus, where there are great differences of temperature between day and night, this agency becomes much more important. During the day the naked rocks are heated very hot by the full blaze of the sun, while at night the rapid radiation which occurs in dry and thin air chills the outer layers of rock very quickly, and they attempt to contract upon the still heated and expanded interior. Thus, stresses are set up which the rock cannot resist, and pieces, great and small, are split off from the surface. In this manner great talus slopes, like those due to frost action, accumulate at the foot of cliffs and on mountain slopes in all dry regions which have hot days and cool nights. Even when the rocks are not shattered to pieces, their crevices and fissures are slowly widened.

Smooth exfoliated surface of granite, Matopos Hills, Rhodesia, South Africa.

Fig. 41. - Smooth exfoliated surface of granite, Matopos Hills, Rhodesia, South Africa.

Certain rocks, notably granites, exfoliate under extreme temperature changes, that is, the surface portions split off in large sheets, which may be of almost any thickness, and are either flat or, more commonly, are curved. In this way are produced the granite domes which are found in so many parts of the world, such as those of the Yosemite, Stone Mountain in Georgia, the Matopos Hills in South Africa. The smooth slopes, due to exfoliation, are often deceptively like those worn and smoothed by glaciers, a resemblance which is heightened by the large boulders, remnants of exfoliated masses, which often occur upon these slopes.

The effect of temperature changes is frequently the disintegration of rocks into minute fragments. This extreme effect is especially noteworthy in those igneous rocks which are coarsely crystalline. A rock of this kind is made up of several different minerals, each of which has its own particular rate of expansion and contraction, and thus the particles are subjected to stresses which gradually separate them and cause the rock to crumble. In Egypt one may pick up granite fragments from the ancient monuments which will break into small pieces upon very slight pressure. The Egyptian obelisk in New York seemed, when first brought to this country, to be perfectly sound and fresh, but the severe winters of our Atlantic seaboard speedily showed how the granite had been rifted and weakened by the centuries of exposure to temperature changes in the dry climate of Egypt.

Slope of exfoliating granite, Matopos Hills.

Fig. 42. - Slope of exfoliating granite, Matopos Hills.

Changes of temperature do work of a purely mechanical kind, as does frost, and are even more entirely superficial than the latter, for a thin covering of debris suffices to put an end to their efficiency.

Exfoliation of glaciated granite, Sierra Nevada. (U. S. G. S.).

Fig. 43. - Exfoliation of glaciated granite, Sierra Nevada. (U. S. G. S.).