]\lAinLAXD Geological Survey 35 



basin between the continental masses as do the oceans and mediterraneans. 

 These seas are illustrated on Plate III, this map being synthetic like the 

 previous one. The various continental seas of Paleozoic time which 

 covered western Maryland, were very shallow, probably never more than 

 two to live hundred feet deep. Into these basins the streams unloaded 

 deposits of great thickness, in Pennsylvania amounting to about 30,000 

 feet during the Paleozoic era. At no time, however, was the Appalachian 

 Sea a deep sea, certainly not in the sense that the Atlantic Ocean is deep, 

 and it is now well known that continental seas subside gently under load- 

 ing, maintaining a fairly constant depth; at times, also, they are com- 

 pletely filled by detrital matter from the land. 



These continental seas are not constant like the oceans, but periodically 

 overflow the land along more or less definite areas. The flood is at 

 first small, but gradually spreads farther and farther until in most cases 

 the waters of two or more oceans become united. The invasion then 

 begins to disappear, accomplishing this far more rapidly than in extending 

 over the land. These phenomena constitute the submergences or trans- 

 gressions and the emergences of stratigraphers, there being eleven such 

 cycles in American Paleozoic formations. 



Some of these floods are due to the subsidence of the land flooded, this 

 being particularly true of the Appalachian Sea., but in most areas there is 

 no movement of the land. The waters of the oceans rise and flow more 

 or less widely over the stationary continents. In great part this may be 

 accounted for by the wearing away of the lands and the distribution of this 

 eroded material in the water areas. Suess has well said : " Every grain 

 of sand which sinks to the bottom of the sea expels, to however trifling a 

 degree, the ocean from its bed." If the present continents above sea-level 

 were unloaded into the ocean, the strand-line would become elevated or 

 positive to the height of 650 feet. Such a displacement of the sea would 

 inundate North America in areal extent not unlike the submergence of 

 the Devonian transgression, illustrated in the paleogeographic map of the 

 Late Hamilton (Plate IX) . 



While the seas and oceans are slowly loading through the wearing away 

 of the rocks of the continents, the internal mass of the earth is also shrink- 

 ing, and the accumulating stresses thus set up finally reach the breaking 



