PHYSIOGRAPHICAL GEOLOGY. 179 



if the earth's nucleus were molten, phenomena similar to ebb 

 and flow would be induced which could only be resisted by a 

 crust of enormous thickness, circa 2000-2800 English miles 

 thick. Besides, if the earth's body were plastic, the oceanic 

 tides would not only be induced by the attraction of the sun 

 and moon, but would also be influenced by deformations of 

 the earth-spheroid. There are, however, no indications of 

 this disturbing influence. Darwin therefore believes that the 

 earth behaves as a rigid body and possesses probably a 

 viscous-elastic constitution. 



Lord Kelvin has essentially the same opinion, and ascribes 

 to the body of the earth a degree of rigidity intermediate 

 between that of steel and of glass. Starting from the nebular 

 theory, Lord Kelvin (1862, 1879) supposes that the cooled 

 and thereby heavier masses sank inward and formed an initial 

 central nucleus, which always extended towards the periphery 

 as the earth's mass continued to cool, until finally almost the 

 whole earth became rigid. Ries and Winkelmann contested 

 (1881) this hypothesis on the ground that not only a number 

 of metals, but also silicate combinations undergo a decrease 

 of density at the moment when they become solid, so that 

 they could not sink in a molten mass. 



The American, Barnard, wrote in 1877 a paper on the 

 internal structure of the earth, considered as affecting the 

 phenomena of precession and nutation. He agreed with 

 Hopkins and Darwin that the behaviour of the earth under 

 the attraction of other bodies in the universe shows a very 

 high coefficient of rigidity for the earth's mass. Reyer in 

 Vienna in the same year brought forward arguments in favour 

 of the theory of rigidity, but supposed that the rigid magma 

 of the nucleus was saturated and impregnated with solvents 

 and gases in so great a degree, that whenever the pressure of 

 the crust was relieved or modified by fractures the nuclear 

 material could readily become viscous or fluid, and capable of 

 eruptive action. 



In opposition to the adherents of the earth's rigidity, many 

 geologists retain the older view, at least in part, in so far as 

 they believe there is a zone of molten magma under the firm 

 crust, and do not accept the extreme conception of the rigidity 

 of the nucleus. 



Sterry Hunt advocated the view that the originally molten 

 globe began to solidify in its central part. At the surface, 



