1SG G. K. GILBERT CONTINENTAL PROBLEMS. 



ize continental elevations, then these elevations would constitute either 

 two polar tracts or else an equatorial belt. Moreover, I have been in- 

 duced by recent studies of the physical history of the moon to suspect 

 that the earth may at one time have received considerable accessions 

 from without, and that these accessions were made to the equatorial tract. 

 If these suspicions are well founded, peculiar characters may have been 

 given to a tract having the form of a belt. So for a double reason I was 

 led to compare the outline of the continental plateau with a great circle- 

 To this end a great circle was chosen, coinciding as nearly as possible 

 with the line of greatest continental extension, and the projection was so 

 •modified as to render the locus of that great circle a straight line. The 

 result appears in figure 5, where the straight line is the projection of the 

 hypothetic ancient equator; and j-ou will probably agree with me that 

 it gives little support to the suggestion that the principal line of conti- 

 nental elevation was originally equatorial. 



Figure 5. — Area of continental Plateau, developed with Reference to a great Circle. 



Why do continental Areas rise and fall f — A fourth problem refers to con- 

 tinental oscillations. The geologic history of every district of the land 

 includes alternate submergence under and emergence from the sea. To 

 what extent are these changes due, on one hand, to movements of the sea 

 and, on the other, to movements of the land, and what are their causes? 

 With American geologists the idea, recently advocated, that the chief 

 movements are those of the ocean finds little favor, because some of the 

 most important of the changes of which we are directly cognizant are 

 manifestly differential. Our paleozoic map pictures a sea where now are 

 Appalachian uplands, and uplands where now are low coastal plains and 

 oceanic waters. In Cretaceous time the two margins of what are now the 

 Great Plains had the same height, or at least the western margin was no 

 higher than the eastern ; but now the western margin lies from four thou- 

 sand to six thousand feet above the eastern, and the intervening rock 

 mass appears to have been gently tilted without important internal dis- 

 tortion. Such geographic revolutions are not to be explained by the 

 shifting of the hydrosphere nor by its dilatation and contraction. Neither 

 can they be ascribed to isostatic restoration of an equilibrium deranged 



