482 C. SCHUCHERT PALEOGEOGRAPHY OF NORTH AMERICA 



bined unloading of the continents into the marine waters, with the added 

 effect resulting from the settling back of the elevated continental borders. 

 Probably there are also other causes, one of which may be the local raising 

 of oceanic bottoms. 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 sealevel were unloaded 

 into the ocean, the strand-line would become positive to the extent of 650 

 feet. Such a displacement of the sea would inundate Xorth America in 

 areal extent and distribution not unlike the submergence caused by the 

 Siluric transgression, the third most extensive flood on this continent. 



With these definitions as a foundation, the various emergences and 

 submergences of the North American continent will now be described. A 

 marked period of emergence combined with one of decided submergence 

 completes a cycle of time, and according to the principle of diastrophism 

 establishes a geologic system or period that may be recognized in all lands 

 affected by inundations. As this subject will be discussed elsewhere, only 

 the systematic names of the new classification will be here introduced. 



Laurentide revolution. — This was one of the "critical periods' 7 of the 

 earth when the seas were withdrawn from North America for a very long 

 time. During this interval, of which only the later or eroding portion is 

 known, the continent was larger than at present, possibly as great as at 

 the close of the Paleozoic, or even greater than at that time. It was a 

 period of time the minimum length of which is measured by the pene- 

 planation of the Unkar-Chuar mountains, Arizona, which were two miles 

 in height, and of other late Proterozoic dislocations. This period was 

 given the name "Laurentide revolution" by Le Conte. 112 He states : 



"The first and by far the greatest of these lost intervals is that which occurs 

 between the Archaean and the Paleozoic. In every part of the earth where the 

 contact has been yet observed the Primordial [== Cambric] lies unconformably 

 on the upturned and eroded edges of the Archaean strata. ... As upturned 

 eroded outcropping strata mean land-surface, it is evident that there was at 

 that time a very large area, or else several large areas of land, in the place 

 now occupied by the American continent. ... It was a continental Period. 

 . . . Was land of the Lost Interval.'''' 



Georgic period or system* — The Laurentide emergent period closed the 

 Proterozoic era and the Paleozoic began with the "Georgian" series. 

 Walcott (1891) demonstrated the existence of a long trough (see plate 

 51) that at this time extended from Alabama northeast to Labrador, 

 being situated on the inner sides of Acadia and Appalachia. In the west 



112 Le Conte : American Journal of Science, vol. 14, 1877, p. 101. 

 * Lower Cambrian and Georgian of most writers. 





