I/O HISTORY OF GEOLOGY AND PALEONTOLOGY. 



wards from the Poles. At first it was merely a thin, flexible 

 rind on the viscous or liquid inner mass. Then the crust 

 while still hot, and locally at a red glow, broke and tore ; the 

 first rains collected in the depressions, and systems of tensions 

 and pressures were generated in consequence of the subsidence 

 of crust-blocks. A more complete phase of movement was 

 reached as the crust became gradually thicker; forces which 

 had during contraction been acting vertically towards the 

 centre were diverted in a tangential direction by the resistance 

 of the crust, and produced the folds and wrinkles represented 

 in our mountain-chains. Continents and oceans also formed, 

 and the crust was in a state to sustain life. In the fourth or 

 final phase, to which the present belongs, the crust has become 

 very thick; cooling and contraction are now proceeding very 

 slowly; the tangential pressures called forth by the sinking 

 crust are relieved by horizontal compression of the rocks at 

 zones and localities of crust-weaknesses. The work done by 

 pressure and fragmentation is converted into heat ; and it was 

 by means of this transmutation that Mallet explained the 

 origin of the earth's own heat, and of volcanoes. 



Mallet's explanation was warmly contested by O. Lang 

 and Julius Roth. Lang differed from most physicists and 

 chemists in his opinion that an increase in volume and not a 

 contraction took place during the transition of the earth's 

 material from the molten into the solid state. He attributed 

 the origin of volcanoes to the expansion of the outer rock- 

 materials during their consolidation and the necessity of 

 additional space. 



Ries and Winkelmann published in 1881 a series of observa- 

 tions on the solidification of melted metals. Their results 

 were so far favourable to Lang's hypothesis in that they 

 proved that, with the exception of cadmium and lead, nearly 

 all other metals are heavier in the molten condition than in the 

 solid. At the same time, Bischofs experiments are contradic- 

 tory, since they prove that the most important plutonic rocks, 

 such as granite, trachyte, basalt, suffer considerable contraction 

 in passing from the molten into the solid state. 



Faye, whose principles of cosmogony were briefly referred to 

 above (p. 155), also made an attempt to explain the origin and 

 development of the earth in agreement both with the doctrines 

 of modern astronomy and with those of geology and palaeon- 

 tology. Starting from his own standpoint that the earth and 



