CENTRAL AMERICAN GEOLOGICAL WATER WAYS. 583 



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 moun- 

 tain ridges about a mile apart. It is the summit of a divide, also about 



Fio. 4. — The Valley at the City of Orizaba, showiiiLf Ijroad terrace plaius bounded by 

 sculptured mountain scenery. 



a mile long. Both ends of the floor of this mountain-bounded de- 

 pression 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 com- 

 pared 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 suf- 

 ficiently old to have what were once the precipitous bounding walls 



