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THE STUDY OF THE STRUCTURE AND FORMATION OF SURFACEFEATURES PRACTICALLY INVOLVES THAT OF THE EVOLUTION OF THE LAND.
IN the preceding chapters we have been inquiring into the origin of surface-features, and have come to the general conclusion that these cannot be accounted for without some knowledge of geological structure. We have learned that the crust of the earth has experienced many changes—rocks have been tilted, compressed, folded, fractured, and displaced. In some places elevation, in other places depression, has taken place, or both kinds of movement have affected the same area at different times. The crust has further been disturbed in many regions by vast intrusions of molten matter; while frequently volcanic action has cumbered the surface with lava and fragmental ejecta. It might seem, therefore, as if the varied configuration of our lands—mountain and valley, height and hollow—might be largely if not exclusively due to subterranean action. But the study of geological structure has shown us that enormous masses of material have been removed from the land-surface, and that however much that surface may have been influenced by crustal disturbance, yet its varied features, as a rule, owe their origin directly to denudation. Great mountain-chains have, indeed, been upheaved from time to time, fractures and displacements have again and again taken place; but even the youngest mountains have been so modified by the various epigene agents of change that frequently their original configuration has been almost completely destroyed. Earth sculpture, in a word, is everywhere conspicuous, and in regions which have remained for long ages undisturbed by subterranean action the latter has had only an indirect influence in determining the form of the surface. All the great ranges of tectonic mountains are of relatively recent age. Time has not yet sufficed for their complete reduction. On the other hand, the mountains that were upheaved in the earlier stages of the world■s history have been either completely remodelled or entirely demolished. If elevations still often mark the sites of the chains and ranges of Palaeozoic times, their internal geological structure yet shows that they are no longer tectonic but relict mountains. In short, we see that epigene agents are constantly endeavouring to remove the irregularities which result from crustal disturbance. Elevations are gradually lowered, and sunken areas filled up. But the process of levelling the land is not infrequently interrupted by renewed crustal movements. No sooner, however, do fresh elevations appear than the cycle of erosion begins again.
"The hills are shadows, and they flow
Although, as a rule, it is not hard to prove that certain surface-features owe their origin to erosion, it is often very difficult, or even impossible, to follow out the whole process—to trace the various stages in the evolution of surface-features. Pyramidal mountains composed of horizontally arranged beds are obviously relict mountains; they have been carved out of horizontal strata. That much anyone can see, and for the student of physical geography it is enough, perhaps, to be able to distinguish such mountains from those of a different build. But a geologist cannot be content with this: he will endeavour to trace out the whole history of the process. He will ascertain, if he can, the age of the strata, and the conditions under which they were accumulated, and subsequently elevated and eroded. It is the story of the evolution or development of the land and its surface-features that he will strive to unfold. In some cases the evidence is so simple, full, and clear, that its meaning can hardly escape him. More frequently, however, it is complicated, incomplete, and hard to read. We may have no doubt whatever that the various surface-features of the region we are examining owe their origin to denudation; but we shall often experience great difficulty in discovering the successive stages through which the land must have passed before it assumed its present configuration. In this volume we have confined attention very much to the simple part of the subject, and have tried to show what kinds of features are due to hypogene and epigene action respectively. Incidentally, however, reference has been made to the successive geological changes which have preceded and led up to existing conditions. It is almost impossible, indeed, to consider the formation of surface-features without at the same time inquiring into their geological history. And not infrequently we find that the configuration of a land is the outcome of a highly involved series of changes. To understand the distribution of its hills and valleys, its plains and plateaux, and the whole adjustment of its hydrographic system, we may have to work our way back to a most remote geological period. But if it be true that the present cannot be understood without a knowledge of the past, it is no less true that physical conditions which have long passed away can often be realised in the existing arrangement of surface-features. This is no more than might have been expected; for if, as we all believe, there has been a continuity in geological history, the germ of the present must be found in the past, just as the past must be revealed in the present, if only we have skill to read the record. Evolution, in a word, is not less true of the land and its features than of the multitudinous tribes of plants and animals that clothe and people it.
This fascinating branch of geology has been followed with much assiduity by many workers in manylands. But it is still in its infancy, and much remains to be accomplished. We have all learned the lesson of denudation. We know that rivers have excavated valleys, that the whole land-surface is being gradually lowered by the activity of the epigene agents. But comparatively few have set themselves the task of working out in all its details the history or evolution of the varied configuration of particular areas. Yet who can look at the map of a well-watered region, a land of mountain and glen, of rolling lowlands and countless valleys, without a wish to trace out the development of its numberless heights and hollows? What a world of interest must often be concentrated in the history of a single river and its affluents! At what time and under what conditions did it first begin to flow? How was its course and those of its tributaries determined? Has the hydrographic system ever been disturbed? and if so, to what extent and in what manner has it been modified? These and many similar questions will come before the investigator, and in searching for answers he must often unfold a strange and almost romantic history.
Naturally investigation of the kind leads up to the larger inquiry—When and how has the land itself been developed? It is matter of common knowledge that the lands within a common area are of very different age. Some have only recently appeared; others are of vast antiquity. And the older ones can always be recognised by the extent to which earth