508 C. SCHUCHERT PALEOGEOGRAPHY OF XORTH AMERICA 



ble throughout the centrosphere or is concentrated chiefly in some portion 

 of the earth. These possibilities, which can not be shown in the tables, 

 increase the error perhaps to 10 per cent, and for the convenience of round 

 numbers the equatorial bulging for each mile of radial shrinkage may 

 therefore be spoken of as from 90 to 100 feet. In latitude 35 degrees the 

 water level will suffer no change. At the poles it will sink some 60 feet, 

 and at the equator it will rise about 35 feet. 



Transgressions. — An analysis of the column in the foregoing table 

 marked "Bate of change of strand-line" will show that of the eleven Pale- 

 ozoic transgressions three moved rapidly over the land. These were the 

 Saint Croix, Eichmond, and Pottsville transgressions. The latter occurred 

 at a time of loaded continental seas and high degrading Appalachia 

 and Acadia. The Eichmond transgression was a rapid duplication of 

 the previous Trenton one, which flowed slowly and widely over a feature- 

 less land. It was seemingly due to the making of some small foreign 

 land, which caused the rapid overflow. The other swift transgression 

 (Saint Croix) was probably more apparent than real, the paleogeographic 

 map of Acadic time as here presented being synthetic and embracing too 

 much time. Two other transgressions were rapid, but were of small areal 

 extent and fell in a time of long persisting emergence. These were the 

 Fern Glen and Saint Louis invasions, which may have owed their swift 

 expansion also to the formation of small extraneous lands. The six re- 

 maining inundations were slow in spreading, these being the Georgic, 

 Ozarkian, Beekmantown, Trenton, Magaran, and Onondaga transgres- 

 sions. The last three were the largest and most significant of all, for 

 they are also well recorded in most other lands, and were true transgres- 

 sions in the sense defined by Suess. These six slow invasions were seem- 

 ingly due to long periods of continental unloading into seas and oceans ; 

 there are no events of sufficient importance known to explain them other- 

 wise. The Ozarkian and Beekmantown inundations were followed by 

 that of the Trenton, and combined they made a more and more extensive 

 transgression marked by two sharp and rapid retreats of the water. 



During the Mesozoic there were four periods of continental seas. The 

 first was the Triassic invasion of the Pacific coast, slow in spreading and 

 culminating in the Upper Triassic. A second appeared and vanished 

 rather rapidly early in the Upper Jurassic, causing the Logan sea. The 

 third began early in the Comanchic, and was of slow spread. A rapid 

 withdrawal of the water then followed, apparently succeeded by an equally 

 swift return of the sea, causing the fourth and much enlarged transgres- 

 sion of early Cretacic time. Beginning in the Niobrara, this flood then 

 slowly retreated, ending with the Laramie series and the Laramide revo- 



