Geographic description will become more and more definite as the observer has more and better type forms to which he may liken those that he finds in his explorations, and the reader, taught from the same types, will gather an intelligent appreciation of the observer's meaning. Take the region north of Philadelphia above referred to. Having grown up upon it, I called it a hilly country, in accordance with the geographic lessons of my school days, and continued to do so for twenty years or more, until on opening my eyes its real form was perceived. It is a surface worn down nearly to a former base level but now diversified by ramifying valleys, cut into the old base level in consequence of a subsequent but not very ancient elevation of a moderate amount. Maturity is not yet reached in the present cycle of development, for there is still much of the old base level surface remaining, into which the valleys are gnawing their head ravines and thus increasing the topographic differentiation. Perhaps not more than a sixth of the total mass above present base level is yet consumed. To say that a country is hilly gives so wide a range to the imagination that no correct conception of it can be gained, but I venture to think that one who understands the terms used can derive a very definite and accurate conception from the statement that a certain country is an old, almost completed base level, raised from one to three hundred feet, and well advanced toward maturity in its present cycle of change.
It is from geographic methods thus conceived that geologic investigation will gain assistance. As the subject is properly developed it will form an indispensable part of the education of every explorer, topographer and geologist; and in its simpler chapters it will penetrate the schools. There is no other subject in which there is greater disproportion between the instruction, as commonly carried on, and the opportunity for application in after life. The intelligent part of the world is travelling from place to place to an extent that our fathers could not have believed possible, and yet not one person in ten thousand has any geographic instruction that enables him to see more than that a river is large or small, or that a hill is high or low. The meaning of geography is as much a sealed book to the person of ordinary intelligence and education as the meaning of a great cathedral would be to a backwoodsman, and yet no cathedral can be more suggestive of past history in its many architectural forms than is the land about us, with its innumerable and marvelously significant geographic forms. It makes one grieve to think of the opportunity for mental enjoyment that is lost because of the failure of education in this respect.
It may be asked perhaps how can one be trained in geographic types, seeing that it is impossible for schools to travel where the types occur. This is surely a great and inherent difficulty, but it may be lessened if it cannot be overcome. Good illustrations are becoming more and more common by means of dry plate photography; maps are improving in number and quality; but the most important means of teaching will be found in models. No maps, illustrations or descriptions can give as clear an idea of relief as can be obtained from a well-made model, and with a set of models, fifty or sixty in number, the more important types and their changes with age can be clearly understood. Maps, illustrations and descriptions supplement the models. The maps should be contoured, for in no other way can the quantitative values be perceived that are essential to good study. The illustrations should be of actual scenes; or, if designs, they should be designed by a geographic artist. The descriptions should wherever possible be taken from original sources, in which the narrator tells what he saw himself. It is, to be sure, not always possible to know what kind of a form he describes, owing to lack of technical terms, but many useful examples can be found that may then be referred to their proper place in the system of geographic classification that is adopted.
I shall consider only one example in detail to show how far short, as it seems to me, geography fails of its great opportunity, both as taught in schools and as applied in after life.
In northeastern Pennsylvania there are several water-falls that leap over tilted beds of rock. Such falls are known to be of rare occurrence, and we may therefore inquire into the cause of their rarity and the significance of their occurrence in the region referred to.
We may first look at the general conditions of the occurrence of water-falls. They indicate points of sharply contrasted hardness in the rocks of the stream channel, and they show that the part of the channel above the fall has not yet been cut down to base level. When the channel reaches base level there can be no falls. Now it is known from the general history of rivers that only a short part of their long lives is spent in cutting their channels down to base level, except in the case of headwater streams, which retain youthful characteristics even through the maturity of their main river. Consequently, it is not likely that at any one time, as now, in the long lives of our many rivers, we should see many of them in their short-lived youthful phase. Falls are exceptional and denote immaturity. They endure a little longer on horizontal beds, which must be cut back perhaps many miles up stream before the fall disappears, than on tilted beds, which must be cut down a few thousand feet at most to reduce them to base level. Falls on tilted beds are therefore of briefer duration than on horizontal beds, and are at any time proportionately rarer. On the headwater branches of a river where youthful features such as steep slope and sudden fall remain after the main river has a well-matured channel, we sometimes find many water-falls, as in the still young branches of the old Ohio. These are like young twigs on an old tree. But even here the rocks are horizontal, and not tilted as in the cases under consideration.
The falls of such headwater streams must persist until the plateau is cut away, for the cap rocks over which the streams leap being horizontal cannot be smoothed down till the whole plateau is cut through. They are long-lived features. Moreover every one of the innumerable branch streams must on its way down from the uplands fall over the outcropping edges of all the hard beds. The falls will therefore be common as well as long-lived features. Their frequent occurrence confirms the correctness of this generalization. On the other hand, in regions of tilted rocks, the hard beds are avoided by the streams, which select the softer strata for their valleys. The hard beds soon stand up as ridges or divides, across which only the large streams can maintain their courses, and these are the very ones that soon cut down any fall that may appear in their early stages. Falls on tilted rocks are therefore rare not only because of their brief duration, but also because tilted rocks are crossed by few streams, except the large ones, which soon cut away their falls.
The foregoing considerations show clearly enough that falls like those of northeastern Pennsylvania are rare, and we have now to consider why they should be prevalent in the region in question. The Appalachians contain many water-gaps cut down on tilted beds, every one of which may have been the site of a fall for a relatively brief period of river immaturity, but this brief period is now left far in the past. The streams show many signs of maturity: their slope is gentle and their valleys are wide open from Alabama to Pennsylvania, but in the northeastern corner of the latter State we find a group of streams that leap over high benches into narrow gorges, and the benches are held up by tilted rocks. Manifestly the streams have in some way been lately rejuvenated; they have been, in part of their courses at least, thrown back into a condition of immaturity, at a time not long past, and, as has so well been shown by White, the cause of this is the obstruction of their old channels by irregular deposits of glacial drift. Here first in the whole length of the Allegheny section of the Appalachians we find an exceptional condition of stream life, and here also we come into a region lately glaciated, where heaps of drift have thrown the streams out of their old tracks. The explanation fits perfectly, and if it had not been discovered by inductive observation in the field, the need of it might have been demonstrated deductively. It is a case that has given me much satisfaction from the promise that it holds out of a wide usefulness for geography, when its forms are systematically studied and its principles are broadly applied.
A final word as to terminology. The material common to geography and geology may be included under the name physiography, as used by Huxley. It is, I think, a subject that is destined to receive much attention. Physical geography, as ordinarily defined, does not cover the ground that it might fairly claim. It is too largely descriptive and statistical. Geographic evolution, as defined by Geikie, is the general preparation of existing geography by geologic processes. It does not consider the general scheme of topographic development or the natural classification of geographic forms.
It is not easy to change the accepted meaning of a term, and I would therefore suggest that a new term should be introduced to include the classification of geographic forms, as advocated here, rather than that any old and accepted term should be stretched over a new meaning. As the essential of the study here outlined is the systematic relation of form to structure, base level and time, the new term might be Systematic Geography.