COSMICAL GEOLOGY, 169 



by Sir William Thomson (Lord Kelvin), in his famous paper 

 "On the Secular Cooling of the Earth" (1862), that even 

 mathematical methods could not lead to any definite calcula- 

 tion of the age of the earth. According to Thomson and 

 Tait's Handbook of Theoretical Physics, the formation of a solid 

 crust took place not less than twenty million years ago, and 

 not more than four hundred million years ago. Helmholtz 

 calculated, upon the basis of the original temperature of the 

 earth-vapour, that the age of the earth might be sixty-eight 

 million years. 



In 1893, the American geologist, Clarence King, published 

 a paper " On the Age of the Earth." He supposes the earth 

 to have been originally molten, and now to have a solid nucleus 

 and a solid crust, and a zone of molten material between crust 

 and nucleus. From a number of observations and experiments, 

 King concludes that the original temperature of the earth was 

 not more than 2000 C., and that its age might be about twenty- 

 four million years. 



A remarkable theory of the earth's constitution was presented 

 by the chemist Sterry Hunt in Canada. He starts from the 

 hypothesis of a homogeneous, gaseous, rotating sphere, in 

 which the parts undergoing condensation seek the centre; 

 there they again become heated, and are kept circulating, 

 finally settling down in zones according to their density and 

 forming a molten, plastic sphere. The consolidation of this 

 sphere begins in the central region. Slow cooling also goes on 

 at the surface of the molten mass, and chemical combinations 

 are effected there owing to the pressure of atmospheric vapours. 

 Gradually a crust forms permeated with water, and in its lower 

 horizons more immediately affected by the internal heat of the 

 earth, the inner crust is again melted and forms a plastic 

 watery zone between the solid, heated nucleus and the outer 

 crust. This intermediate zone is the centre of volcanic action, 

 of earthquakes, and of deforming changes in the earth's crust. 



Another ingenious thinker in this subject was Robert Mallet 

 (1810-81), a civil engineer in Dublin. Mallet thought that 

 the cooling of the original molten sphere began at the Poles. 

 Certain portions, as they solidified at the Poles, sank into the 

 molten mass, but again rose to the surface at the equatorial 

 regions and began to return towards the Poles, the circulation 

 of rock -material being analogous with that of the ocean currents 

 at the present day. The formation of a crust proceeded out- 



