Page:Popular Science Monthly Volume 78.djvu/246

236 that lies between the subdued limestone range and the long radial slopes of the volcanoes; perhaps they rise in lakes; but not until they are all confluent is an outlet found across the broad and low saddle between the third and fourth volcanoes; this being the lowest saddle presumably because these two volcanoes stand farthest apart. There the united waters of the rivers from the back country cut down a transverse consequent valley roughly a hundred meters in depth, open it to mature width, and prograde a simple cuspate delta in the sea beyond. At the sides of the main valley, the spurs between the radial consequent streams of the neighboring volcanoes are cut off by the river, and frayed out by insequent wet-weather streams into small hills of similar form and subequal height, consisting of tuff lying on the clays of the prevolcanic lowland (or sea bottom), and here, with the subdued Sabine range of the Apennines in the background (northeast) and the blue waters of the Tyrrhene sea in the foreground, on a few of these frayed out hills, not signalized otherwise from their fellows, the Eternal City was built; these hills are the Seven Hills of Rome.

Three or four minutes may be required for this introductory statement. The various specifications introduced in these few minutes—subdued mountains of deformed limestones; large volcanic cones, with calderas of engulfment replacing their original summits, and radial consequent valleys submaturely dissecting their gentle outward slopes; a consequent river, traversing the sag of a broad saddle between two neighboring volcanic slopes, eroding a mature consequent valley, and prograding a simple cuspate delta—all these specifications are easily understood by hearers who are ready to listen to explanatory regional descriptions. The relative positions of the several features may be indicated by a blackboard diagram, or by a lantern slide made from a pen and ink drawing, and are all so easily conceived that it is not really necessary to point even once to the diagram as the successive elements of the landscape are mentioned. At the end of the three minutes the hearers will have grasped the essential features of the district about Rome. Then a second and fuller statement of the same facts may be begun, from which the hearers may learn that the limestones of the Sabine mountains seem to be of subequable resistance, for the bare domes of the subdued range have rounded forms of coarse texture, without distinct exhibition of structural trends in the ridges or valleys; that there are many small irregularities in the course of the consequent valleys of the volcanic slopes, previously described as of radial arrangement, a geometrical phrase that suffices very well as a first approximation to the fact, but which thus suffices only because it serves as a good beginning for a closer approximation; that the longitudinal river in the depression between the limestone range and the volcanoes receives three branches from the back country, the northernmost and largest bearing the Tiber name to its head in the valleys of the central Apennines,