Page:The National geographic magazine, volume 1.djvu/30

 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 topographic 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 has 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 systematic 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 mark the second stage in the advance of scientific topography, referred to above.