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.
It is not easy to sketch the history of this awakening. Ramsay years ago contributed an element in his explanation of plains of marine denudation; Jukes opened the way to an understanding of cross valleys; Newberry excluded fractures from the production of the most fracture-like of all water ways; and our government surveyors in the western territories have fully developed the all important idea of base level, of which only a brief and imperfect statement had previously been current. I cannot say how far European geographers and geologists would be willing to place the highest value on the last named element; to me it takes the place of Lesley's ocean flood, in leading off the whole procession of outdoor facts. It is indispensable at every turn. Recently, mention should be made of Löwl, of Prague, who has done so much to explain the development of rivers, and of McGee, who has explicitly shown that we must "read geologic history in erosion as well as in deposition."