Notices of Memoirs — Hutton, on Mountains. 



169 



Fahr., the mean temperature of the old surface would be raised 2°, 

 or to 52°, while at 100 feet below it would be 54'^, and so on, so that 

 the covering of the surface by a deposit 100 feet thick would raise 

 the temperature of the whole underlying rocks 2°. If the deposit 

 was thicker, the temperature would of course be more raised in pro- 

 portion. Now we know that rocks expand on being heated, ' and 

 conti'act on being cooled, and Colonel Totten and Mr. Adie have 

 shown that this expansion for each degree of temperature is from 

 Tys-o'o ^^ To""s-n 0" '-'^ '•^^® whole, according to the nature of the rock. 

 If, however, the deposit was unconsolidated, like clay or sand, and 

 the particles were free to move among themselves, this expansion 

 would have very little effect in raising the surface ; but if the deposit 

 was rigid, like limestone, the effect would be totally different, and 

 the irresistible pressure, caused by the expansion of the rock, could 

 only be relieved by the whole stratum bulging upwards and forming 

 an arch, or more properly a dome ; and as we know the rate of 

 expansion, we can calculate what the elevation would have to be on 

 a sphere the size of the earth, for various temperatures and for 

 different areas, in order to relieve the pressure. This is exhibited in 

 the following table, which is part of a larger table that I have calcu- 

 lated.^ In it the upper line is the thickness in feet of the deposit, 

 while the second line is the temperature due to that thickness. The 

 left-hand column is the diameter, or breadth, in miles of the heated 

 area, while the other columns show the elevation in feet that would 

 take place : 



Thickness 



500 feet. 



2,500 feet. 



10,000 feet. 



25,0€0 feet. 



Temperature 



10" 



50" 



200' 



500° 



Breadth, 100 miles ... 



■500 „ ... 



„ 1,000 ,. ... 



„ 2,000 „ ... 



1,140 feet 

 1,550 „ 

 1,570 „ 

 1,900 „ 



3,700 feet. 

 7,220 „ 

 7,700 „ 



7,800 ,y 



8,700 feet. 

 ; 24,200 „ 

 28,600 „ 

 30,700 „ 



14,600 feet 

 49,300 „ 

 65,400 „ 

 74,400 „ 



From this table it will be seen that formations no thicker nor more 

 extensive than those that we know to have been deposited, are quite 

 capable of being elevated far above the highest known mountains. 



It may have occurred to you that a bed of limestone would not be 

 capable of supporting itself as an arch, and, therefore, that instead of 

 being elevated it would break up into fragments ; this is very true, 

 if the arch was entirely unsupported, but as soon as the expansion 

 overcame the rigidity of the crust, and movement commenced, the 

 underlying superheated rocks, being relieved from pressure, would 

 rise up and still press upwards on the rising arch, so that the 

 pressure expended in elevation would be that capable of overcoming 

 the rigidity only of the crust, and not its weight. You may also 

 have noticed that unless the rate of deposition was greater than the 



1 Clay contracts on being heated, but this does not affect the theory. 

 ^ This table is calculated on the suppositions that the earth is a sphere, with a 

 radius of 3956 miles, and that rocks expand -000005 for 1° Fahr. 



