EARTH'S SURFACE METEORIC ABRASION. 79 



under its influence, the rock alternately expanding 

 and contracting, and thereby disintegrating more 

 rapidly there than in the land that has a northern 

 aspect, where the changes in temperature are less. 

 The different degrees of weathering due to aspect are 

 very apparent during frost; for at mid-day, if a 

 bank or cliff looks south, and there is a strong sun, 

 numerous blocks and fragments will be loosened and 

 drop away, while in a bank or cliff only a few yards 

 distant, but having a northern aspect, not even a 

 fragment will be displaced. This may be studied in 

 artificial cuttings, such as those for roads the north 

 bank yearly weathering away, while years must 

 elapse before the south side loses the form given it 

 by the workmen. The effects of the sun's rays may 

 also be traced in a bog which, during dry weather, 

 cracks with the heat, as in general the principal 

 cracks are nearly perpendicular to the sun's rays a 

 little before noon. 



The sun's heat may have opposite effects under 

 different circumstances. Generally, heat will expand 

 the matter on which it acts. This, however, may be 

 sometimes not the case with rocks, as the component 

 strata of the earth's crust contain more or less 

 moisture, which the sun's heat evaporates, thus 

 causing the rocks to contract; but at the same time 

 there would be at the first an expansion due to the 

 heating of the moisture prior to its evaporation* 



