1866.] 



Position of the Axis of the Earth's Crust. 



53 



Taking the increment of heat as 1° Fahrenheit for every 55 or 60 feet* 

 in descent, a temperature of 2400° Fahr. would be reached at about 25 miles, 

 sufficient to keep in fusion such rocks as basalt, greenstone, and porphyry ; 

 and such a thickness appears much more consistent with the fluctuations in 

 level, and the internal contortions and fractures of the crust which are 

 everywhere to be observed. Sir William Armstrong, on the assumption of 

 the temperature of subterranean fusion being 3000° Fahr., considers that 

 the thickness of the film which separates us from the fiery ocean beneath 

 would be about 34 miles. 



Even assuming a thickness of 50 miles, so as to make still greater 

 allowance for the increased difficulty of fusion under heavy pressure, the 

 thickness of the crust would only form one-eightietb part of the radius of 

 the earth; or if we represent the earth by a globe 13 feet in diameter, 

 the crust would be one inch in thickness, while the difference between the 

 polar and equatorial diameters would be half an inch. 



In such a case, the elevation or wearing away of continents such as are 

 at present in existence, rising, as some of them do, nearly a quarter of a 

 mile on an average above the mean sea-level, would cause a great dis- 

 turbance in the equilibrium of the crust, sufficient to overcome considerable 

 resistance in its attempts to regain a state of equilibrium by a movement 

 over its fluid nucleus. 



Whether the thickness of the earth's crust was not in early geological 

 times less than at present, so as to render it more susceptible of alterations 

 in position — whether the spheroid of the fluid mineral nucleus corresponds 

 in form with the spheroid of water which gives the general contour of 

 the globe — whether or no there are elevations and depressions upon the 

 nucleus corresponding to some extent with the configuration of the outer 

 crust, and whether the motion of the crust upon it, besides effecting 

 climatal changes, might not also lead to some elevations and depressions of 

 the land, and produce some of the other phenomena mentioned by Sir 

 Henry James, are questions which I will leave for others to discuss. 



My object is simply to call attention to what appears to me the fact, 

 that if, as there seems reason to suppose, our globe consists of a solid crust 

 of no great thickness resting on a fluid nucleus, either with or without a 

 solid central core, and if this crust, as there is abundant evidence to prove, 

 is hable to great disturbances in its equilibrium, then it of necessity follows 

 that changes take place in the position of the crust with regard to the 

 nucleus, and an alteration in the position of the axis of rotation, so far as 

 the surface of the earth is concerned, ensues. 



Without in the shghtest degree undervaluing other causes which may 

 lead to climatal changes, I think that possibly we may have here a vera 

 causa such as would account for extreme variations from a Tropical to an 

 Arctic temperature at the same spot, in a simpler and more satisfactory 

 manner than any other hypothesis. 



* Page, ' Advanced Text-book of Geology,' p. 30. 



