National Geographic Magazine/Volume 1/Number 1/Geographic Methods in Geologic Investigation

by W. M. Davis

GEOGRAPHIC METHODS IN GEOLOGIC INVESTIGATION.

By W. M. Davis.

OUTLINE.

Definition of Geography and Geology—Geographic Methods in Geology—Hutton and Lyell Marine deposits explained by existing processes reveal the history of the earth—American Topographers—First Pennsylvania Survey; geographic form as the result of extinct processes—Western Surveys; geographic form explained by existing processes reveals the history of the earth—Deductive Topography—Comparison with Palæontology—Geographic Individuals—Classification according to structure—Ideal cycle of regular development—Interruptions in the Simple Ideal Cycle—Geography needs ideal types and technical terms—Comparison with the biological sciences—Teaching of Geography—The water-falls of Northeastern Pennsylvania as examples of deductive study—Systematic Geography.

 

The history of the earth includes among many things an account of its structure and form at successive times, of the processes by which changes in its structure and form have been produced, and of the causes of these processes. Geography is according to ordinary definition allowed of all this only an account of the present form of the earth, while geology takes all the rest, and it is too generally the case that even the present form of the earth is insufficiently examined by geographers. Geographic morphology, or topography, is not yet developed into a science. Some writers seem to think it a division of geology, while geologists are as a rule too much occupied with other matters to give it the attention it deserves. It is not worth while to embarrass one's study by too much definition of its subdivisions, but it is clearly advisable in this case to take such steps as shall hasten a critical and minute examination of the form of the earth's surface by geographers, and to this end it may serve a useful purpose to enlarge the limited definition of geography, as given above, and insist that it shall include not only a descriptive and statistical account of the present surface of the earth, but also a systematic classification of the features of the earth's surface, viewed as the results of certain processes, acting for various periods, at different ages, on divers structures. As Mackinder of Oxford has recently expressed it, geography is the study of the present in the light of the past. When thus conceived it forms a fitting complement to geology, which, as defined by the same author, is the study of the past in the light of the present. The studies are inseparable and up to a certain point, their physical aspects may be well followed together, under such a name as physiography. Specialization may then lead the student more to one subject than to the other.

An illustration from human history, where the study of the past and present has a single name, may serve to make my meaning clear in regard to the relation of the two parts of terrestrial history, which have different names. A descriptive and statistical account of a people as at present existing, such as that which our statistical atlas of the last Census gives in outline, corresponds to geography in its ordinary limitation. A reasonable extension of such an account, introducing a consideration of antecedent conditions and events, for the purpose of throwing light on existing relations, represents an expanded conception of geography. The minute study of the rise and present condition of any single industry would correspond to the monographic account of the development of any simple group of geographic forms. On the other hand, history taken in its more general aspects, including an inquiry into the causes and processes of the rise and fall of ancient nations, answers to geology; and an account of some brief past stage of history is the equivalent of paleography, a subject at present very little studied and seemingly destined always to escape sharp determination. It is manifest that geology and geography thus defined are parts of a single great subject, and must not be considered independently.

History became a science when it outgrew mere narration and searched for the causes of the facts narrated; when it ceased to accept old narratives as absolute records and judged them by criteria derived from our knowledge of human nature as we see it at present, but modified to accord with past conditions.

Geology became a science when it adopted geographic methods. The interpretation of the past by means of a study of the present proves to be the only safe method of geologic investigation. Hutton and Lyell may be named as the prominent leaders of this school and if we admit a reasonable modification of their too pronounced uniformitarianism, all modern geologists are their followers. The discovery of the conservation and correlation of energy gives additional support to their thesis by ruling out the gratuitous assumption of great results from vague causes. Causes must be shown to be not only appropriate in quality, but sufficient in quantity before they can be safely accepted. But the geographic argument as expounded by the English school deals almost entirely with processes and neglects a large class of results that follow from these processes. Much attention is given to the methods of transferring the waste of the land to the sea and depositing it there in stratified masses, from which the history of ancient lands is determined. But the forms assumed by the wasting land have not been sufficiently examined. It was recognized in a general way that land forms were the product of denudation, but the enormous volume of material that had been washed off of the lands was hardly appreciated, and the great significance of the forms developed during the destruction of the land was not perceived.

Hutton says a little about the relation of topography to structure; Lyell says less. The systematic study of topography is largely American. There is opportunity for it in this country that is not easily found in Europe. The advance in this study has been made in two distinct steps: first, in the East about 1840; second, in the West about 1870. The first step was taken in that historic decade when our early State surveys accomplished their great work. The Pennsylvania surveyors then developed topography into a science, as Lesley tells us so eloquently in his rare little book "Coal and its Topography," 1856, which deserves to be brought more to the attention of the younger geographers and geologists of to-day. It presents in brief and picturesque form the 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 their true arrangement. The face of nature thus becomes alive and full of expression, and the conception of its change becomes so real that one almost expects to see the change in successive visits to one place.

Now consider the deductive application of this principle. Having recognized the sequence of forms developed during the wasting life of a single structure, reverse the conception and we have a powerful geographic method for geologic investigation. On entering a new country, apply there the principles learned from the inductive study of familiar regions, and much past history is revealed; the age of mountains may be deduced from their form as well as from their rocks; the altitudes at which a district has stood may be determined by traces of its old base levels, of which we learn nothing from the ordinary routine of geologic observation, that is, from a study of the structure and age of the rocks themselves. The principle is commonly employed nowadays, but its methods are not formulated, and its full value is hardly yet perceived. Heim has found traces of successive elevations in the Alps, proved by incipient base levels at several consistent altitudes on the valley slopes. Newberry, Powell and Dutton have worked out the history of the plateau and cañon region from its topography; Chamberlin and Salisbury write of the young and old topographic forms of the drift-covered and the driftless areas in Wisconsin; LeConte and Stephenson have interpreted chapters in the history of California and Pennsylvania from the form of the valleys. Recently McGee has added most interesting chapters to the history of our middle Atlantic slope, in an essay that gives admirable practical exposition of the geographic methods. In the light of these original and suggestive studies one may contend that when geographic forms in their vast variety are thus systematically interpreted as the surface features of as many structures, belonging to a moderate number of families and having expression characteristic of their age and accidents, their elevation and opportunity, then geography will be for the wasting lands what palæontology has come to be for the growing ocean floors.

An interesting comparison may be drawn here. Fossils were first gathered and described as individual specimens, with no comprehension of their relationships and their significance. It was later found that the fossils in a certain small part of the world, England—that wonderful epitome of geologic history—were arranged in sequences in the bedded rocks containing them, certain groups of forms together, successive groups in shelves, as it were, one over another. Then it was discovered that the local English scale had a wider application, and finally it has come to be accepted as a standard, with certain modifications, for the whole world. The exploring geologist does not now wait to learn if a formation containing trilobites underlies another containing ammonites, but on finding the fossils in the two, confidently and as far as we know correctly concludes that such is their relative position. Thus the sequence of submarine processes is made out by the sequence of organic forms. In brief, palæontology has passed largely from the inductive to the deductive stage.

The geographer first regarded the features of the land as completed entities, with whose origin he was in no wise concerned. Later it was found that some conception of their origin was important in appreciating their present form, but they were still regarded as the product of past, extinct processes. This view has been in turn displaced by one that considers the features of the land as the present stage of a long cycle of systematically changing forms, sculptured by processes still in operation. Now recognizing the sequence of changing forms, we may determine the place that any given feature occupies in the entire sequence through which it must pass in its whole cycle of development. And then reversing this conception we are just beginning to deduce the past history of a district by the degree of development of its features. Geography is, in other words, entering a deductive stage, like that already reached by palæontology.

The antecedent of deductive topography is the systematic study of land geography. The surface of the land is made up of many more or less distinct geographic individuals, every individual consisting of a single structure, containing many parts or features whose expression varies as the processes of land sculpture carry the whole through its long cycle of life. There is endless variety among the thousands of structures that compose the land, but after recognizing a few large structural families, the remaining differences may be regarded as individual. In a given family, the individuals present great differences of expression with age, as between the vigorous relief of the young Himalaya and the subdued forms of the old Appalachians; or with elevation over base level, as between the gentle plain of the low Atlantic coast and the precocious high plateaus of the Colorado river region; or with opportunity, as between the last named plateaus with exterior drainage and the high plains of the Great Basin, whose waters have no escape save by evaporation or high level overflow; or with complexity of history, as between the immature, undeveloped valleys of the lava block country of southern Oregon, and the once empty, then gravel-filled, and now deeply terraced inner valleys of the Himalaya. When thus studied, the endless variety of the topography will be considered in its proper relations, and it will not seem as hopeless as it does now to gain a rational understanding and appreciation of geographic morphology.

We should first recognize the fact that a geographic individual is an area, large or small, whose surface form depends on a single structure. Boundaries may be vague, different individuals may be blended or even superposed, but in spite of the indefiniteness, the attempt to sub-divide a region into the individuals that compose it will be found very profitable. In a large way the Appalachian plateau is an individual; the Adirondacks, the terminal moraine of the second glacial epoch are others. In a small way, a drumlin, a fan delta, a mesa, are individuals. The linear plateaus of middle Pennsylvania are hybrids between the well-developed linear ridges of the mountains farther east and the irregular plateau masses farther west.

A rough classification of geographic individuals would group them under such headings as plains, plateaus, and rough broken countries of horizontal structure; mountains of broken, tilted or folded structure, generally having a distinct linear extension; volcanoes, including all the parts from the bottom of the stem or neck, up to the lateral subterranean expansions known as laccolites, and to the surface cones and flows; glacial drift; wind drift. The agents which accomplished the work of denudation are also susceptible of classification: rivers according to the arrangement of their branches, and their imperfections in the form of lakes and glaciers. The valleys that rivers determine may be considered as the converse of the lands in which they are cut; and the waste of the land on the way to the sea is susceptible of careful discrimination: local soil, talus, alluvial deposits, fan cones and fan deltas, flood plains and shore deltas. Their variations dependent on climatic conditions are of especial importance. The structures formed along shore lines are also significant. This list is intentionally brief, and the lines between its divisions are not sharply drawn. It undoubtedly requires discussion and criticism before adoption. It differs but slightly from the common geographic stock in trade, but for its proper application it requires that the geographer should be in some degree a geologist.

The changes in any geographic individual from the time when it was offered to the destructive forces to the end of its life, when it is worn down to a featureless base level surface, are worthy of the most attentive study. The immaturity of the broken country of southern Oregon, as compared with the more advanced forms of the Basin ranges, is a case in hand. The Triassic formation of the Connecticut valley is in some ways of similar structure, being broken by long parallel faults into narrow blocks or slabs, every block being tilted from its original position. Russell's description of the blocks in southern Oregon would apply nicely to those in Connecticut, except that the former have diverse displacements, while the latter all dip one way; but the Connecticut individual has, I feel confident, passed through one cycle of life and has entered well on a second; it has once been worn down nearly to base level since it was broken and faulted, and subsequent elevation at a rather remote period has allowed good advance in a repetition of this process. The general uniformity in the height of its trap ridges and their strong relief above the present broad valley bottom, require us to suppose this complexity of history. A given structure may therefore pass through two or more successive cycles of life, and before considering the resulting composite history in its entirety, it would be best to examine cases of simple development in a single cycle. After this is accomplished, it would be possible to recognize the incomplete partial cycles through which a structure has passed, and to refer every detail of form to the cycle in which it was produced.

The most elementary example that may be chosen to illustrate a simple cycle of geographic life is that of a plain, elevated to a moderate height above its base level. The case has already been referred to here and is given in more detail in an article printed in the proceedings of the American Association for the Advancement of Science, for 1884, to which I would now refer. When the succession of forms there described as developed at a given elevation over base level is clearly perceived, the occurrence of forms dependent on two different base levels in a single region can easily be recognized. The most striking example of such a complex case that I know of is that of the high plateaus of Utah, as described by Dutton. Northern New Jersey presents another example less striking but no less valuable: the general upland surface of the Highlands is an old base level, in which valleys have been cut in consequence of a subsequent elevation. The plateau developed on the tilted Triassic beds about Bound Brook is a second base level, cut during a halt in the rise from the previous lower stand of the land to its present elevation. There is a parable that illustrates the principle here presented.

An antiquary enters a studio and finds a sculptor at work on a marble statue. The design is as yet hardly perceptible in the rough cut block, from which the chisel strikes off large chips at every blow; but on looking closer the antiquary discovers that the block itself is an old torso, broken and weather beaten, and at once his imagination runs back through its earlier history. This is not the first time that the marble has lain on a sculptor's table, and suffered the strong blows of the first rough shaping. Long ago it was chipped and cut and polished into shape, and perhaps even set up in its completed form in some garden, but then it was neglected and badly used, thrown over and broken, till its perfect shape was lost, and it was sold for nothing more than a marble block, to be carved over again if the sculptor sees fit. Now it just beginning its second career. We may find many parallels to this story in the land about us, when we study its history through its form. The sequence of events and consequently of forms is so apparent here that no one could have difficulty in interpreting history from form, and it shall come to be the same in geography. The gorge of the Wissahickon through the highland northwest of Philadelphia can have no other interpretation than one that likens it to the first quick work of the sculptor on the old torso.

An essential as well as an advantage in this extension of the study of geography will be the definition of types and terms, both chosen in accordance with a rational and if possible a natural system of classification. Types and terms are both already introduced into geographic study, for its very elements present them to the beginner in a simple and rather vague way: mountains are high and rough; lakes are bodies of standing water, and so on. It is to such types and terms as these that every scholar must continually return as he reads accounts of the world, and it is to be regretted that the types are yet so poorly chosen and so imperfectly illustrated, and that the terms are so few and so insufficient. Physical geography is particularly deficient in these respects, and needs to be greatly modified in the light of the modern advance of topography General accounts of continental homologies of course have their interest and their value, but they are of the kind that would associate whales with fishes and bats with birds. The kind of reform that is needed here may be perceived from that which has overtaken the biological sciences. The better teaching of these subjects lays representative forms before the student and requires him to examine their parts minutely. The importance of the parts is not judged merely by their size, but by their significance also. From a real knowledge of these few types and their life history it is easy to advance in school days or afterwards to a rational understanding of a great number of forms. Few students ever go so far in school as to study the forests of North America or the fauna of South America. It is sufficient for them to gain a fair acquaintance with a good number of the type forms that make up these totals. It is quite time that geography should as far as possible be studied in the same way. No school boy can gain a comprehensive idea of the structure of a continent until he knows minutely the individual parts of which continents are composed. No explorer can perceive the full meaning of the country he traverses, or record his observations so that they can be read intelligently by others until he is fully conversant with the features of geographic types and with the changes in their expression as they grow old. Both scholar and explorer should be trained in the examination and description of geographic types, not necessarily copies of actual places, before attempting to study the physical features of a country composed of a large number of geographic individuals. When thus prepared, geography will not only serve in geologic investigation, it will prosper in its proper field as well.

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.