DYNAMICAL GEOLOGY, 323 



the sedimentary rocks beside a rupture are tilted by upheaval, 

 then, according to Reyer, the rock-strata glide downward and 

 as they do so fall into complicated folds. 



An Alpine geologist of wide experience, Professor Rothpletz 

 in Munich, holds the Contraction Theory to be inadequate as 

 an explanation of volcanoes and of the unlike distribution 

 of gravity in the earth's crust. He believes that a better 

 explanation is afforded on the basis of crust-expansion in 

 certain regions. 



Rothpletz recognises three distinct spherical zones of rock- 

 material in the earth, according to their physical condition. 

 Below the rigid crust is the viscous or molten nucleus, and 

 between both a zone of cooling and consolidation. Professor 

 Rothpletz assumes that the masses in the intermediate zone do 

 not contract as they would on cooling under normal pressure of 

 superincumbent rock, but expand as they cool, in analogy with 

 bismuth and other substances. From this zone, therefore, 

 vertical and tangential pressures are exerted upon the solid 

 crust. At localities of weak resistance the crust is torn, the 

 expansion of the intermediate zone pushes the crust upward 

 and produces continents or table-lands at the surface, and the 

 seams are invaded by the uprush of molten magma from the 

 nucleus. At the same time the tangential tension in the 

 emerged continents tries to relieve itself locally by the forma- 

 tion of folds. Hence mountains are upheaved and volcanic 

 invasions occur on the continents at the places of least 

 resistance. 



