EPICONTINENTAL SEDIMENTATION 489 



Furthermore, if the continents had grown by sedimentation along 

 their margins, one would expect to find deep-sea sediment at 

 the base of the sedimentary structure. However, it is absent in 

 ancient geosynclines. Finally, the continents are much too small to 

 have been growing throughout geological time at the rate indicated 

 by the present terrace. 



If one were to postulate repeated terrace building followed by 

 vertical collapse, that would require the assumption of so many 

 revolutions for which there is no supporting evidence and against 

 which geophysicists are bound to protest. Can it be, then, that 

 the terraces are sapped from the outside by erosion, for instance 

 by turbidity currents? This process would imply a huge volume of 

 sediment carried to the ocean floor, more than anyone is prepared 

 to admit as still lying there. 



Turning to the Gulf Coast for enlightenment, we are confronted 

 with a terrace of normal appearance. Heie sedimentation has 

 been even more spectacular in volume. Only modest, if any, gain 

 in surface area for the continent has been attained at the cost of 

 dumping shallow-water sediments to a thickness of 5000 m. Again 

 only a few per cent of geological time is involved. A solution for 

 this difficulty might be sought by pointing to the abnormally high 

 average level of present continents. In the Paleozoic and Mesozoic, 

 large sections were usually flooded by the sea. In the Tertiary 

 gradual retreat of the oceans became more obvious, and at present 

 the continents stand with their average level hundreds of meters 

 above sea level. This must cause abnormal denudation and sedi- 

 mentation, and in former ages the terraces may have grown much 

 more slowly. This viewpoint is no great help, because all we can 

 do is to reduce the number of times former terraces must have 

 vanished from 25 to perhaps 5, but we cannot explain them all 

 away. 



The mystery is only aggravated by the fact that at several places 

 along the borders of the continents geologists have found cogent 

 evidence for land supplying vast amounts of sediment from out- 

 side the present continental margins. An attempt has been made 

 to squeeze in a source in the shape of an island arc with volcanoes 

 between the geosynclinal trough with sedimentary fill and the 



