EFFECT OF CHANGING TEMPERATURES 25 



temperatures facilitate chemical action, rocks are more readily 

 decomposed by the chemical action of the atmosphere in warm than 

 in cold regions. Changes of temperature tend to disrupt rock, and 

 thus increase the amount of rock-surface exposed to chemical 

 change. The elements of the atmosphere are much more active 

 chemically in moist than in dry regions. 



Though the chemical changes effected by the air are slow, their 

 importance in the course of the earth's long history has been very 

 great. The amount of rock which has been thus disintegrated 

 probably far exceeds all that is now above the sea. 



Till: ATMOSPHERE AS A CONDITIONING AGENCY 



Temperature effects. Changes of temperature tend to break 

 up rocks. The heating of rock by day and its cooling by night 

 produce some such change in it as is produced by the quick heat- 

 ing and cooling of glass. When the surface of the rock is heated, it 

 expands, and a strain is set up between the hotter and more expand- 

 ed part at the surface, and the cooler and less expanded part below. 1 

 This strain is enough to make the surface of the rock shell off in 

 many cases. Daily variations in temperature are much more 

 important than yearly variations, because they are much more 

 common and take place more suddenly. Variations which do not 

 involve the freezing of water are more important in long periods of 

 time than those which do, because they are so very much more 

 common. The daily range of temperature is influenced especially 

 by (i) latitude, (2) altitude, and (3) humidity, (i) If other things 

 were equal, the greatest daily ranges of temperature would be in 

 low latitudes. (2) High altitudes favor great daily ranges of tem- 

 perature, so far as the rock surface is concerned, for though the rock 

 becomes heated during the sunny day, the thinness and dryness of 

 the atmosphere allow the heat to radiate rapidly at night. Here, 

 too, the daily range of temperature is likely to bring the wedge- 

 work of ice into play. Since the south side of a mountain (in the 

 northern hemisphere) is heated more than the north, it is subject 

 to the greater daily range of temperature, and the rock on this side 

 suffers the greater disruption. Similarly, rock surfaces on which 

 the sun shines daily are subject to greater disruption than those 



1 It is the change of temperature of the rock surface, not the change of temper- 

 ature of the air above it, which is considered here. 



