170 CONTINENTAL PROBLEMS OF GEOLOGY. 



low coastal plains and oceanic waters. In Cretaceous time the two 

 marijins of what are now the Great Plains had the same height, or at 

 least the western margin was no higlier than the eastern ; but now the 

 western margin lies from four thousand to six thousand feet above the 

 eastern, and the intervening rock mass api)ears to have been gently 

 tilted with(mt im])<ntant internal distortion. Such geographi*' revo- 

 lutions are not to l)e exjdained by the shifting of the hydro-sphere nor 

 by its dilatation and contraction. N"either can they be ascribed to iso- 

 static restoration of an eijuilibrium deranged through the transfer of 

 masses by erosion and sedimentation, for that liypothetic process i.« 

 essentially conservative. Neitlier is it easy to believe that the two mar- 

 gins of the plains have differed, since the Cretaceous, to the extent of 

 one mile in their radial contraction due to secular cooling of the globe; 

 nor is it easy, at least for the disciple of isostasy, to believe that such 

 a change can have resulted from the localization of deformation con- 

 sequent on the slowing of the earth's rotation. Each of these processes 

 may have been concerned, but I conceive that the essential factor still 

 awaits suggestion. Our knowledge of surface processes, as compared 

 to subterranean, is so full that the field of plausible epigene hypothe- 

 ses may be exhausted, but the vista of hypogene i)ossibility still opens 

 broadly. 



Are continents permanent? — The doctrine of the permanence of the 

 continental plateau, enunciated long ago by Dana and more recently 

 advocated, with a powerful array of new data, by Murray and Wallace, 

 has made rapid progress toward general acceptance. Nevertheless its 

 course is not entirely clear, and among the obstacles still to be over- 

 come is one whose magnitude is perhaps magnified for the American 

 student by proximity. All who have studied broadly the stratigraphy 

 of the Appalachian district have concluded that the sediments came 

 chietly from the east, and the detailed Appalachian work of the past 

 decade is disclosing a complicated history, in which all chapters tell of 

 an eastern paheozoic land and some chapters seem to testify to its wide 

 extent. At some times the western shore of this land lay east of the 

 sight of the Blue Ridge, and there is serious doubt w^hether the exist- 

 ing belts of coastal plain and submerged continental shelf afford it 

 sutticient space. For the present, at least, the subject of continental 

 permanence must be classed with the continental problems. 



Do continents (fro IV f — According to my own view^ there is yet another, 

 a sixth, continental ])roblem deserving the attention of the World's 

 Fair inter-continental congress. We have been told by the masters of 

 our science (and their teaching has been echoed in every text-book and 

 in every classroom), that through the whole ])eriod of the geologic 

 record the continents have grown; not that the continental plateaus 

 have been materially extended, not that the pendulum has moved 

 always in one direction, but that the land area has on the whole 

 steadily increased. From this doctrine there has been no dissent — 



