460 A TREATISE ON METAMORPHISM. 



takes place. Thus it appears that at certain seasons the temperature is 

 high enough for decomposition to affect the rocks in the most northern 

 and the most southern latitudes. But in polar regions the temperature is 

 such that decomposition takes place for only a small fraction of each year; 

 in the temperate regions the temperature is high enough for decomposition 

 to take place during' more than half the year; while in the Tropics decom- 

 position is continuous. It therefore appears that in passing from the polar to 

 the tropical zones we pass from a minimum to a maximum of chemical 

 decomposition. 



(c) As to the pressure in the belt of weathering, since the ground is 

 not ordinarily saturated by water there is commonly no hydrostatic pres- 

 sure; and the pressure at which the chemical reactions take place is there- 

 fore that of the atmosphere. At sea level the average pressure is that 

 of a column of mercury 760 mm. high, or 1.0333 kg. per sq. cm. From the 

 pressure at sea level the pressure diminishes as the altitude increases, and 

 at the tops of the higher mountains is not more than about one-half of the 

 amount at sea level. Not only is there change of pressure due to elevation, 

 but there is change of pressure due to variable meteoric conditions. As a 

 storm sweeps over an area the pressure is ordinarly low; at the clear inter- 

 vals between the storms the pressure is usually high. Commonly the 

 change in barometer from a low to a high is not more than 2 cm. and the 

 average change is probably not more than 1 cm. Infrequently the change 

 is 5 cm., or about one-fifteenth of the, total pressure. Very exceptionally 

 the change of pressure may be much greater than this. Probably these 

 changes in pressure are so slight as to produce little effect upon the water 

 solutions. But, as already shown (p. 61), the quantity of atmospheric gas 

 acting is directly as the pressure. To illustrate, if the barometric pressure 

 'rises from 715 mm. to 760 mm., the proportional increase in the amount of 

 gas action is about one-fifteenth. More frequently, however, the variations 

 are from one-thirtieth to one-fiftieth, or even less. 



Since the amount of gas is directly as the pressure, it is evident that, 

 so far as this factor goes, the higher the barometer the greater the activity 

 of the gases at work in the belt of weathering. But since the variations in 

 pressure due to this cause are ordinarily but a small fraction of the total 

 pressure, the variations in effect are not sufficiently great to be appreciable. 

 Since, in reference to the cyclonic periods, the times of high pressures are 



