ORIGIN AND AGE OF THE SUNS HEAT. 293 



what appeared to be a very obvious objection to this 

 argument, viz., that the influence of sub-aerial denuda- 

 tion in altering the form of the earth had been over- 

 looked. It has been proved, as we have seen, that the 

 rocky surface of our globe is being lowered, on an 

 average, by sub-aerial denudation, at the rate of about 

 1 foot in 6000 years. It follows as a consequence, 

 from the loss of centrifugal force resulting from the 

 retardation of the earth's rotation occasioned by the 

 friction of the tidal wave, that the sea-level must be 

 slowly sinking at the Equator and rising at the Poles. 

 This, of course, tends to protect the polar regions, and 

 expose equatorial regions to sub-aerial denudation. 

 Now, it is perfectly obvious that unless the sea-level 

 at the Equator has, in consequence of tidal retardation, 

 been sinking during past ages at a greater rate than 

 1 foot in 6000 years, it is physically impossible the 

 form of our globe could have been very much different 

 from what it is at present, whatever may have been 

 its form when it consolidated, because sub-aerial denu- 

 dation would have lowered the Equator as rapidly as 

 the sea sank. But in equatorial regions the rate of 

 denudation is no doubt much greater than 1 foot in 

 6000 years, because there the rainfall is greater than 

 in the temperate regions. It has been shown ('Climate 

 and Time,' p. 336) that the rate at which a country 

 is being lowered by sub-aerial denudation is mainly 

 determined not so much by the character of its rocks 

 as by the sediment-carrying power of its river systems. 

 Consequently, other things being equal, the greater the 

 rainfall the greater will be the rate of denudation. 

 We know that the basin of the Ganges, for example, 

 is being lowered by denudation at the rate of about 

 1 foot in 2300 years ; and this is probably not very far 

 from the average rate at which the equatorial regions 



