172 



DISCOVERY 



though critically, the results of many investigations 

 and creates a higher synthesis of his own. Old facts 

 appear, but in a new light ; halting half-truths gain a 

 new relevancy and live in a larger and wider meanmg. 

 The conception is so vast in scope and involves such 

 a universe of detail that here we can give the merest 

 outline, though no outline can do justice to it. 



number of relatively firm blocks separated from each 

 other by zones of less solidified and more plastic 

 material. The geological history of the earth is 

 occasioned by the interaction of these two fundamental 

 elements. 



The blocks arc to be thought of as great table-like 

 expanses, of irregular outline, worn flat ^ through age- 



^ Boiindanes of the great 

 structural (continental} units 



-yiAP SHOWING MAIN STRUCTURAI, FEATURES OF THR EARTH. 

 {Adapted from L. Kobcr : " Der Ban dcr Erde.") 



The dominant cause of movements of the earth's 

 crust is earth contraction due to earth cooling. Other 

 causes (astronomical, geophysical) undoubtedly oper- 

 ate, but they are neither so constant nor so general. 

 Other causes, again, are derivative, secondary, or later 

 results of the great primary cause, shrinkage. 



When it emerged from the astronomical into the 

 geological stage the earth had a crust composed of a 



long erosion. They must lie at approximately one 

 general relative level which is (for their surfaces) very 

 nearly the mean general elevation of the land-surfaces 

 of the globe. The Russian, Siberian, Australian 

 Tables and the Canadian Shield are examples. These 

 old blocks arc composed of intensely crumpled, 



' Really convex, according to the curvature of the earth's 

 surface. 



