STRAND-LINE DISPLACEMENTS 481 



seas, causing them either to spill over and inundate the other lands or caus- 

 ing their bottoms to subside. The strand-line is thus constantly affected 

 either in a positive or a negative manner. In fact, this alternate loading 

 and sinking explains the irregularity, at least, in the oscillatory nature of 

 all continental seas. During a period of loading the waters are more and 

 more displaced and submerge wider areas of land. If no subsidence of 

 the sea-bottom takes place the basin will eventually become completely 

 filled and all its water dispersed and added to other marine seas, thus 

 causing the strand-lines to become positive. During a period of subsi- 

 dence, however, the water is naturally contracted, and for a time parts of 

 the former littoral region are exposed. In this way the strand-lines of 

 aggrading seas are continually affected and made slightly positive or 

 negative. 



Again, if an area which is rapidly loading in vast quantities is con- 

 stantly subsiding, there will result either some isostatic compensation in 

 the way of land-making elsewhere, or where there is no compensation the 

 entire adjacent areas will be dragged down. In the latter case, if such 

 an area of a continental sea is close to the ocean the land barrier will be 

 submerged, allowing communication between the two marine bodies of 

 water. This is thought to have been the case during the deep subsidence 

 of the New York basin, where every now and then the Atlantic has access 

 to the Appalachian trough. In this region, either periodic isostatic com- 

 pensation appears to have been operative or the tangential thrusting of 

 the Atlantic ocean has repeatedly renewed the land barriers, which in the 

 end were not only eroded, but again dragged down and submerged. 

 Throughout the Paleozoic there was almost constant subsidence in the 

 Mexico embayment, and the isostatic compensation must therefore have 

 been farther removed than in the case of the New York basin. The 

 Mississippian sea is an excellent example of an aggrading sea. 



The "transgressive seas" are likewise aggrading seas, yet they owe 

 their distinctive character not to minor local fillings, but to the combined 

 deposits of all marine waters ; also to the united effects of all the isostatic 

 compensation having an upward movement, such as land-making and the 

 elevation of the ocean-bottom as well. 



Transgressing continental seas. — These bodies of water, which are due 

 to a general eustatic elevation of the strand-lines (eustatic positive strand- 

 lines), are the great continental seas that more or less simultaneously 

 affect all continents. They are slow in attaining their maximum expan- 

 sion, but vanish fairly rapidly following the periodic shrinkage of the 

 earth, which naturally exerts more influence on the oceanic areas than 

 on the lands. The migratory reexpansion of these seas is due to the com- 



