topographical results of the first geological survey of Pennsylvania. It shows how Lesley and the other members of that survey "became not mineralogists, not miners, not learned in fossils, not geologists in the full sense of the word, but topographers, and topography became a science and was returned to Europe and presented to geology as an American invention. The passion with which we studied it is inconceivable, the details into which it leads us were infinite. Every township was a new monograph." (p. 125.) Some of the finest groups of canoes and zigzags developed on the folded beds of the Pennsylvania Appalachians are illustrated from studies made by Henderson, Whelpley and McKinley, and they certainly deserve the most attentive examination. I often feel that they have been of the greatest assistance in my own field work, especially in the efforts I have 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.

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."

If it be true that the greater part of this second advance is American like the first, it must be ascribed to the natural opportunities allowed us. The topographers of the Appalachians had a field in which one great lesson was repeated over and over again and forced on their attention. The patchwork structure of Europe gave no such wide opportunity. The surveyors of the western territories again found broad regions telling one story, and all so plainly written that he must run far ahead who reads it. It is to this opportunity of rapid discovery and interpretation that Archibald Geikie alludes in the preface to the recent second edition of his charming volume on the "Scenery of Scotland." He says that since the book first appeared he has seen many parts of Europe, "but above all it has been my good fortune to have been able to extend the research into western America, and to have learned more during my months of sojourn there than during the same number of years in the Old Country." (p. vii.)

Our position now is, therefore, while structure determines form as our earlier topographers taught, and while form-producing processes are slow, as had been demonstrated by the English geologists, that the sequence of forms assumed by a given structure during its long life of waste is determinate, and that the early or young forms are recognizably different from the mature forms and the old forms. A young plain is smooth. The same region at a latter date will be roughened by the channeling of its larger streams and by the increase in number of side branches, until it comes to "maturity," that is to the greatest variety or differentiation of form. At a still later date the widening of the valleys consumes the intervening hills, and the form becomes tamer, until in "old age" it returns to the simple plain surface of "youth." Young mountains possess structural lakes and are drained largely by longitudinal valleys; old mountains have no such lakes and have transverse drainage, formed as the growing headwaters of external streams lead out much water that formerly followed the longitudinal valleys. Young rivers may have falls on tilted beds, but such are short lived. Falls on horizontal beds are common and survive on the headwater branches of even mature rivers. All falls disappear in old rivers, provided they are not resuscitated by some accident in the normal, simple cycle of river life. The phases of growth are as distinct as in organic forms. As this idea has grown in my mind from reading the authors above named, geography has gained a new interest. The different parts of the world are brought into natural relations with one another; the interest that change, growth and life had before given to the biologic sciences only, now extends to the study of inorganic forms. It matters not that geographic growth is destructive; it involves a systematic change of form from the early youth to the distant old age of a given structure, and that is enough. It matters not that the change is too slow for us to see its progress in any single structure. We do not believe that an oak grows from an acorn from seeing the full growth accomplished while waiting for the evidence of the fact, but because partly by analogy with plants of quicker development, partly by the sight of oaks of different ages, we are convinced of a change that we seldom wait to see. It is the same with geographic forms. We find evidence of the wasting of great mountains in the wasting of little mounds of sand; and we may by searching find examples of young, mature and old mountains, that follow as well marked a sequence as that formed by small, full grown and decaying oaks. If the relative positions of the members in the sequence is not manifest at first, we have the mental pleasure of searching for