Page:The National geographic magazine, volume 1.djvu/34

 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