14 National Geographic Magazine. 



made to discover the structural arrangement of the Triassic lava 

 sheets in the Connecticut valley. But although the intricacies of 

 Appalachian topography were then clearly seen to depend on the 

 complications of Appalachian structure, the process of topo- 

 graphic development was not at that time discovered. "The 

 only question open to discussion is," says Lesley, " whether this 

 planing down of the crust to its present surface was a secular or 

 an instantaneous work" (p. 132), and he decides in favor of the 

 latter alternative. He adds, that to the field worker, " The rush 



of an ocean over a continent leads off the whole procession 



of his facts, and is indispensable to the exercise of his sagacity at 

 every turn" (p. 166). "The present waters are the powerless 

 modern representatives of those ancient floods which did the 

 work" (p. 151). 



It is not the least in any spirit of disparagement that I quote 

 these cataclysmic views, now abandoned even by their author. 

 Great generalizations are not often completed at a single step, 

 and it is enough that every effort at advance should have part of 

 its movement in the right direction. What I wish to show is 

 that 'topographic form was regarded in the days of our eastern 

 surveys, even by our first master of American topography, as a 

 completed product of extinct processes. Topography revealed 

 structure, but it did not then reveal the long history that the 

 structure hae passed through. The anticlinal valleys, hemmed in 

 by the even-topped sandstone mountains of middle Pennsylvania, 

 were found to tell plainly enough that a vast erosion had taken 

 place, and that the resulting forms depended on the structure of 

 the eroded mass, but it was tacitly understood that the land stood 

 at its present altitude during the erosion. The even crest lines of 

 the mountains and the general highland level of the dissected 

 plateau farther west did not then reveal that the land had stood 

 lower than at present during a great part of the erosion, and thus 

 the full lesson of the topography was not learned. The system- 

 atic relation of form to structure, base level and time ; the 

 change of drainage areas by contest of headwaters at divides ; 

 the revival of exhausted rivers by massive elevations of their 

 drainage areas : all these consequences of slow adjustments were 

 then unperceived. In later years there seems to be a general 

 awakening to the great value of these principles, which mai-k the 

 second stage in the advance of scientific topography, referred to 

 above. 



