504 C. SCHUCHERT PALEOGEOGRAPHY OF NORTH AMERICA 



Thus there are minor and major emergences, and many more of the 

 former than of the latter. All are thought to have the value of periods 

 or systems, while the major ones arrange the minor beats into eras. Be- 

 cause of the wide distribution of the transgressions whose deposits are 

 concordantly laid down, it is thought that total and actual continental 

 uplift, followed by subsidence, can not be the explanation for these emer- 

 gences. It must therefore be assumed that the continents are in the main 

 unmoved, and that it is the oceanic areas which are periodically depressed, 

 owing to the shrinkage of the earth, the result being an apparent eleva- 

 tion of the lands. The continents are chiefly horsts, unmoved during the 

 periods of altered strand-lines. It may also be noted that the greater 

 number of oceanic subsidences are relatively quick to appear, and that the 

 emergence thus established is apt to be soon invaded by another transgres- 

 sion. Further, there are in America either five, four, or three of these 

 minor emergences to one major one, as follows: 



Georgic 

 Franconia 

 Shakopee 

 Saint Peter 

 Utica 



Cayugan ] Appalachian Tr ia«sic ^ 



/laconic Chemung [revolution -, '/• ! Laramide 



revolution Keokuk ((includes p 1,Ckfel ,- (revolution 



Kaskaskia J Perinic) U>mancnic J 



It is well known that the emergent period represented by the Lauren- 

 tide revolution .closing the Proterozoic was also of very long duration, but 

 it is not known whether it was likewise of slow origin. At the close of 

 each revolution the emergence of the !N"orth American continent was com- 

 plete and the areal extent of the land in each case was about that of today, 

 possibly even larger. The relative duration and importance of these revo- 

 lutions may be expressed by the following order: (1) Laurentide, (2) 

 Appalachian, (3) Laramide, and (4) Taconic. 



The minor oceanic subsidences serve to divide geologic time into periods 

 or systems. Their rhythm beats with equal regularity on the positive and 

 negative strand-lines. During the early "Ordovicic" note the influence 

 of the small Shakopee and Saint Peter emergences on the long persisting 

 positive strand-line ; also the effect of the small Chemung, Keokuk, and 

 Kaskaskia emergences on the equally long negative strand-line of the 

 Devonic and Mississippic. 



The major oceanic subsidences gather, as it were, the minor beats into 

 the eras, the critical periods of the earth. These major times of earth 

 shrinkage and stress discharge are long in duration. During such times 

 the continental borders are being remodeled into new frames of mountain 

 ranges. The continents are then largest and remain longer emergent, as 

 may be seen by the Laurentide, Taconic, Appalachian, and Laramide revo- 



