The Evolution of the Universe 



21 



of life on the earth. Others, however, beheve that the drifting has 

 been much more recent and still continues (Wegener, 1924; DuToit, 

 1937; Runcorn, 1959; and Irving, 1959). 



The hypothesis that continents occupy fixed positions rests chiefly 

 on interpretations of the folding of the crust resulting in island arcs 

 and linear mountain ranges associated with adjacent deep troughs 

 and geosynclines. Geosynclines are usually relatively narrow but 

 long depressions (Figs. 7, 8) which gradually fill with sediments 

 washed in from adjacent highlands. As they fill, the geosynclines 



Fig. 7. Diagrammatic representation of a geosyncline. 1, original depression; 

 2, depression filling with sediments washed in from the adjacent highlands and 

 simultaneously sinking; 3, sedimentary rocks of geosyncline elevated into 

 mountains. Umbgrove depicts lateral compression accompanying these steps. 

 (After Umbgrove, The Pulse of the Earth, by permission of Martinus NijhoflF.) 



