62 Rev. 0, Fisher — Formation of Mountains. 



(1) The area of the ocean is 146 millions of square miles. 



(2) That of the land is 51 millions. 



(3) The mean depth of the ocean is four miles. 

 . (4) Its deepest parts are about five miles. 



(5) The mean height of the land is 900 feet (as shown by Mr. 

 Carrick Moore).' 



From these data I have obtained as a probable value 2 ite =: 11000 

 feet, which appears to me more likely to be too small than too large. 



The meaning of this in plain language is, that if all the inequali- 

 ties of the earth's surface were levelled down, they would form a 

 coating 11,000 feet thick over the whole globe above the datum 

 level ; the datum level being such a surface as has been already 

 defined. 



Having thus obtained a value for the thickness of the coating 

 which all the inequalities of the earth's surface would foi-m, if 

 levelled down, I was led to seek for a measure of the same thing on 

 physical grounds. For this purpose I availed myself of Sir W. 

 Thomson's paper " On the Secular Cooling of the Earth," as a basis 

 to work from.^ From Mr. R. Mallet's late investigations on the con- 

 traction of slag from an iron furnace,^ I deduced a probable coeffi- 

 cient of contraction for melted rock, viz. 0O000217 for 1° Far. ; and 

 with this I obtained a value for 2 Ice, or the thickness of the coating 

 above defined. Sir W. Thomson's investigation proceeds upon the 

 supposition, founded upon Bischoff 's experiments upon the contrac- 

 tion of melted rocks in cooling, that, if the earth, or an outer coating 

 of it, were once in a molten state, then, as soon as a crust began to 

 form, it would break up and sink, and thus the whole would be re- 

 duced to the temperature of incijoient solidification before it could be 

 permanently crusted over. From the time of such incipient solidifi- 

 cation it has gone on cooling, subject to the laws of cooling of a solid. 



He then proves that upon this supposition the temperature would 

 increase from the surface downwards, at first at a nearly uniform 

 rate, but at a greater depth much more slowly, until at a certain 

 point such a temperature would be arrived at, as would be about 

 sufficient to induce fusion under the pressure existing at that depth. 

 Now the rate at which the temperature first begins to increase is 

 known to be about 1° Far. for 51 feet. Sir W. Thomson has deter- 

 mined, by observation on the rocks at Edinburgh, that their con- 

 ductivity on an average is 400. With these data he proves that if, 

 for the sake of illustration, the temperature at which the crust began 

 to solidify be taken at 7000° Far., then the time since such solidifi- 

 cation commenced will have been about one hundred millions of 

 years, and that at about 100 miles below the surface the melting 

 temperature would be reached. 



Proceeding upon these assumptions, with the coefficient of con- 

 traction for rock above mentioned, I have calculated the value of 2 A; e, 

 or the thickness of the coating which all the elevations would form if 

 they were levelled down, and I find it to come out less than 800 feet. 



1 Nature, 1872, vol. v. p. 479. 



2 Edin. Trans. 1862; and Natural Philosophy, p. 711. 



3 Royal Soc. Trans. 1873. 



