. Bulletin of the Geological Society of America. Geology; Geology -- United States. 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.
. Bulletin of the Geological Society of America. Geology; Geology -- United States. 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 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 ocea
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