Page:Popular Science Monthly Volume 53.djvu/40

 brought the physical geography of that region into its present form. While the presence of man upon the West Indian and eastern American plateaus is not even suggested by remains of him, yet at that time there roamed over the savannas and through the forests several animals identical with and many closely related to the existing species; and this is as near as we may hope to connect the sunken lands with human associations.

The application of the deep-sea channels to the measurements of the subsidence of the land is a recent innovation. The elevation of mountain regions to great heights has long since passed beyond the bounds of doubt, but their forms have only recently been used for interpreting their history. The reason for this lies in the fact that many of the necessary topographical and geological explorations have not been made until recent years, and because geologists were chiefly engaged upon problems of greater antiquity than modern land features, especially on fossil remains which told something of the history of the sea basins; so that their interest in these questions caused them to largely overlook the history of the land.

If we look at the Alpine plateau surmounted by the sublime Matterhorn, we may be told by geologists that, somewhere in the Alps, the Tertiary formations have been found at heights of ten or twelve thousand feet; in the Himalaya Mountains similar marine strata occur to an elevation of twenty thousand feet. We are still left in ignorance of the geographical history of the region, in spite of the geological youthfulness of the formations. Since the comparatively recent Tertiary period much of the molding of the physical features has been effected. If we observe the Matterhorn, as shown in Plate IV, any one may see a gently sloping plain dissected by a valley thousands of feet deep, which has almost passed the cañon stage, but is yet very immature. Now the geomorphist or scientific geographer will almost at a glance interpret the story recorded in these forms. The plateau is the remains of a base level of erosion, and bears testimony of its formation at a level of nine thousand or ten thousand feet lower, or near sea level, and the marine fossils found in the district will tell since what geological epoch the low plains were formed which have since become elevated into the high plateau. The Matterhorn itself, in spite of the faults, is largely a remnant of a higher mountain mass, which was worn down to a base-level floor by the insidious action of the rains and rills. The valley bears record of an elevation so recent, in spite of the hardness of the rocks and the slowness of degradation, that it has not yet entirely passed its youthful stage. The actually slow excavation of such a great valley, since the last geological period preceding the modern, certainly impresses us with the enormous