Page:Popular Science Monthly Volume 53.djvu/603

Rh throughout a long period, in wearing them down to near sea level. The remains of such elevated plains were observed at an altitude of even ten thousand six hundred and thirty-six feet above the sea, as shown in Fig. 3. This represents a level plain bounded by mountain ridges about a mile apart. It is the summit of a divide, also about



a mile long. Both ends of the floor of this mountain-bounded depression are open, and terminate in abrupt steps of three hundred feet or more to the next lower and very much greater plains. The margins of this flat summit are indented by short ravines of recent date (the character of which is illustrated in Fig. 6, page 585), that have not yet had time to be extended backward into the summit plain and to dissect it into sharp ridges and deep ravines.

Equally abrupt are the margins of the plateau back of Vera Cruz (at about eight thousand feet above the sea), which may be taken as a type of the high base levels of Mexico. The borders of the high plateaus are dissected by deep valleys, which, when compared with the extent of the elevated plains, are remarkably short—often only a few miles long. The distance between Esperanza, upon the edge of the plateau (at an altitude of eight thousand and forty-two feet), and Atoyac (at fifteen hundred and twelve feet), near where the valley opens on the coastal plain, is only about forty miles in a direct course. The valley is from one to two or more miles in width, and is of course very deep. Upon its broad terrace plains the city of Orizaba and other large towns are situated, as shown in Fig. 4. Compared with the age of this large, deep valley, which is sufficiently old to have what were once the precipitous bounding walls