Proceedings of the Ohio State Academy of Science 37 



This would mean that there has been no permanence of 

 ocean and continent through the past, that they have been inter- 

 changeable. Present opinion however is that continents and 

 deeo sea have held in general their present position since the be- 

 ginning of Paleozoic time, and probably much longer. This is 

 not to deny that at times the present continents have been widely 

 submerged beneath ocean waters, for this has happened again 

 and again. But these epicontinental seas, as they have been 

 called, have been shallow, and the real continental border has 

 been at the edge of the shallow circumcontinental terrace, where 

 the relatively steep slope to the oceanic abyss commences. The 

 continental seas have been the overRowings of the over-full 

 ocean onto the continental platform, so that a modern geological 

 editor, sacrificing beauty to truth, might change Tennyson's lines 

 to read : 



There where the long street roars, hath been 

 An epicontinental sea. 



One reason for this belief in the permanence of continents 

 is the fact that the marine deposits now showing in the rocks of 

 the continent are essentially shallow water deposits. Even lime- 

 stones indicate clear rather than deep water. Another line of 

 evidence, developed more recently, lies in the fact, determined by 

 gravity measurements and plumb-line deflections, that the earth 

 mass beneath the continents is of less specific gravity than be- 

 neath the oceanic basins. Indeed these measurements show that 

 a practical equilibrium exists between the oceanic and continental 

 segments of the earth, the greater length of radius of the con- 

 tinental segments being compensated for by their less specific 

 gravity, the masses of the two being the same, area for area. If 

 continent and ocean basin are in this isostatic adjustment, the 

 great difference of relief between continental platform and ocean 

 basin is evidently an expression of difl^ering density beneath the 

 surface. It is difficult to see how the difiference of density could 

 essentially change, and we have therefore a permanent cause of 

 difference between continent and ocean. 



This fact of the essential permanence of continental platform 



