THE ELEMENTS OF A PALEOGEOGRAPHIC PROBLEM 45 



however the upheaval may have been accomplished. There is here a hint 

 which may guide the paleobiologist in postulating land connections. Such 

 ridges as are necessary should be drawn or searched for parallel to old lands 

 or upon their edges. The old edges of the North Atlantic Continent are 

 still traceable in the seacoast of Great Britain, France, and eastern Canada. 

 Such evidence does not appear for Gondwana Land. 



In examining some paleogeographic maps, as those drawn by Scharf, 

 we find long bridges drawn parallel to the coast, as from the middle of 

 western South America to the Galapagos Islands and north. Such sugges- 

 tions seem at first to exceed the possibilities, as the geologist knows them, 

 but there is a possible explanation in the conceptions of Wegener that the 

 continents have decreased in size by constant contraction and elevation, 

 and such ridges may have existed and parts been left behind as the major 

 portion receded inward. Again, it is possible that previously exposed areas 

 parallel to existing continental blocks have been obliterated in whole or in 

 part by the "suboceanic shove" discussed by Ulrich, which he would 

 demonstrate by the inward position of repeated uplifts in the geosynclinal 

 region of the Appalachian Mountains. Opposed to such a conception is the 

 idea of continental creep elaborated by Chamberlin ; but as one force works 

 outward from above and the other inward from below y there would be a 

 constant, if intermittent, movement down and in at the edge of the con- 

 tinental block which would not preclude the existence, temporarily at least, 

 of parallel lands bordering the present blocks. Such conceptions are far 

 more feasible than ridges flung boldly across what we know to have been 

 permanent ocean basins. 



Such broad questions are, however, only to be hinted at in a summary 

 outline. Only extended consideration will permit the true weighing of the 

 evidence. The author has found Dacque, "Grundlagen and Methoden der 

 Palaogeographie," and Grabau, " Principles of Stratigraphy," excellent intro- 

 ductions to the literature of this subject. 



