Elevation and Subsidence of the Surface of the Earth. 407 



for each foot^ no land would rise from this cause above the sea- 

 level ; for as the temperature would then increase as fast as the 

 deposition, as soon as the latter stopped the former would stop 

 also. This, according to Sir W. Thomson^s calculations (" On 

 the Secular Cooling of the Earth/' Trans. Roy. Soc. Edin. 1862), 

 would have happened about 11,500,000 years after the cooling 

 of the crust, or some 88,500,000 years ago ; and from that time 

 to the present, elevations must have been on an increasing scale. 



If also the interior heat travels outward at from 16 to '1 inch 

 per year, it would take from 46,200 to 60,000 years to advance 

 500 feet, which is equal to an increase of temperature of 10° F. 

 Now from the Table we find that an increase of temperature of 

 10° F. implies an elevation of 1140 feet if the heated area was 

 100 miles in diameter, or of 1900 feet if its diameter was 2000 

 miles; so that in the first case an elevation of 1140 feet would 

 have taken place in from 46,200 to 60,000 years, or at the rate 

 of from 1'9 to 2*44 feet per century, while in the latter case the 

 elevation would have been 1900 feet in the same time, or at the 

 rate of from 3*17 to 4' 11 feet per century. We may therefore 

 conclude that elevation from this cause proceeds at a rate of 

 from 2 to 4 feet per century. 



Professor Geikie and Mr. CroU have shown that subaerial 

 denudation is a very slow process, requiring from 700 to 

 12,000 years to remove a foot in thickness, so that at pre- 

 sent it will hardly affect the elevation ; but as the earth cools, 

 the radiation and conduction outwards will be slower, and 

 consequently elevation will be slower also ; and a time will 

 come when it will not exceed the rate of denudation, and when 

 consequently no land can be formed, and the earth will again 

 relapse into a quiescent globe surrounded by water*. If we 

 take the least estimate of denudation, or 1 foot in 12,000 years, 

 as that which will be nearest to the truth when the land is 

 nearly all flat, it follows that the elevation, if it just keeps pace 

 wdth the denudation, will have to be only g^^Q of its present rate, 

 or the interval of temperature must increase only at the rate of 

 — — 1-— ° F. per foot. This, according to Sir W. Thomson's theory, 

 will not take place until about 13 billions of years have passed; 

 consequently the earth is as yet only in its infancy between birth 

 and the repose of old age. 



Before proceeding any further with these speculations, it will, 

 I think, be advisable to give an illustration of this theory taken 

 from nature. 



In his celebrated paper on the structure of the Wealden 



* Unless indeed the oxidation of the interijr had before this time ab- 

 sorbed the water and the oxygen from the air, thus leaving a bald earth 

 surrounded by a thin atmosphere of nitrogen, which is not hkely. 



