480 C. SCHUCHERT PALEOGEOGRAPHY OF NORTH AMERICA 



With each period of marked sinking of the oceanic areas the strand-line 

 becomes negative and everywhere recedes to a lower level around the con- 

 tinental horsts. The lands then appear to stand higher. The continents 

 therefore attain their elevation through two causes : ( 1 ) Low lands over 

 vast areas, due to the negative eustatic character of the strand-line, and 

 (2) a more or less high altitude resulting locally from the tangential lat- 

 eral thrusts of the oceans or vertical movements due to isostatic readjust- 

 ments. The smaller invasions made b}^ the sea may therefore be caused 

 by lands produced by tangential thrusts, such movements being apt to 

 form synclines along the inner sides away from the oceans, or the water 

 may vary in local distribution, owing to its being attracted by the land- 

 masses. Further, as the detrital material from the land is unloaded irreg- 

 ularly and locally into the continental seas, the submergences may be 

 locally accentuated. There are, therefore, various types of continental 

 seas, and these may be named and denned as follows : 



Attracted continental seas. — During times of decided emergence due to 

 the greater altitude and extent of the land-masses, the oceans may be 

 drawn up the sides of these elevations several hundred feet, thus causing- 

 their margins or preexisting depressions to be flooded. Such seas are met 

 with after periods of actual elevation or eustatic negative movements. 

 The resulting seas are small, and such are thought to be most frequently 

 present in the Saint Lawrence sea. 



Synclinal continental seas. — During recurrent periods of unrest the 

 oceans thrust the margins of the continents inward and away from their 

 areas. In the early stages of such movements broad and low folds are pro- 

 duced, which together make synclinoria along the inner sides of the mass 

 thus disturbed, the folds being most numerous, higher, and closer together 

 toward the ocean. Therefore the deepest continental troughs appear imme- 

 diately at the base of such lands ; the sea flows into them and makes long 

 but narrow waterways. In the developmental stages of such a synclino- 

 rium the sea is at first apt to be broader and shallower. As the folds are 

 successively accentuated by the subsequent thrusting, the water areas not 

 only become deeper, but also more complex; hence a series of troughs 

 may finally appear that may or may not be in communication with one 

 another. The Appalachian and Saint Lawrence seas, with their sinking 

 Lenoir, Chazy, and Levis troughs, are good examples of such seas. The 

 Appalachian and Great Basin synclines had each finally subsided in cer- 

 tain local areas to a maximum of at least 30,000 feet. 



Aggrading continental seas. — Being decidedly the areas of loading, con- 

 tinental seas are therefore aggrading seas; they are rarely degrading. 

 Great quantities of detrital matter are transferred by the rivers to these 



