7 6 EVOLUTION OF THE EARTH 



continents were largest and at some of these times were even 

 larger than they are now. On the other hand, the present 

 oceans are more than full and for some time have been spilling 

 over the outer edges of the continental platforms, apparently 

 to the extent of nearly ten million square miles or about 5 per 

 cent of the earth's surface. In other words, the present con- 

 tinental platforms occupy 35.5 per cent of the lithosphere, 

 while the total water areas are equal to 69.6 per cent of the 

 earth's surface. 



The mean height of the present continents above the sea is 

 given by Sir Archibald Geikie as about 2,400 feet, and if these 

 protuberant masses were deposited in the sea, as they surely 

 will be by the future rivers, the ocean waters would be dis- 

 placed to the extent of raising the strand-line about 650 feet. 

 We see therefore that the ocean level is also inconstant, and 

 accordingly, even though the North American continent should 

 remain stationary, about half of it would be covered by the 

 displaced seas to depths of several hundred feet. 



Between the comparatively short intervals of mountain 

 making and crustal unrest occur the long quiet times of erosion 

 when the lands are planed down to near the sea-level. This 

 transference of rock leads, as has already been stated, to more 

 or less flooding of the continents by the oceans (see Fig. 12). 

 During the Paleozoic, Mesozoic, and Cenozoic eras, North 

 America, and especially the United States, was widely flooded 

 by warm and shallow marine waters at least sixteen times 

 Waucobian, Croixian, Canadian, Chazyian, Mohawkian, 

 Cincinnatian, Silurian, Devonian, Mississippian, Pennsylva- 

 nian, Permian, Triassic, Jurassic, Comanchian, Cretaceous, 

 and in an oscillatory and marginal way during the Cenozoic 

 (see Fig. 13). These shallow-water spreadings begin in a 

 small way, grow to greater dimensions throughout a very long 

 time, and then, declining, recede more rapidly than they came 

 (see Fig. u). They vary in areal extent up to 4,000,000 



