584 



POPULAR SCIENCE MONTHLY. 



of a great canon now rounded off into sloping mountains, the Mexi- 

 can plateau, of hundreds of miles in extent (at an altitude of eight 

 thousand feet more or less), which is dissected for a distance of only 

 forty miles, is of great age. Here again the elevation of the table- 

 land has been too recent to have been converted into rugged moun- 

 tain ridges and deep valleys. 



This Mexican valley back of Vera Cruz, whose grandeur and 

 beauty are perhaps unsurpassed, and depend more or less upon its 

 youthful features, is not exactly modern, for since its first develop- 

 ment it became partly refilled with muds and gravels, which the 

 streams are again removing. The slope of its floor is not uniform, 

 but it is represented by great series of gradation plains bounded by 

 abrupt steps, as shown in Fig. 5. These steps may be only five or 

 ten feet in height, or several may coalesce into one great terrace. 

 Three of the greater steps are perhaps five hundred feet above the 

 floor below. The valley heads abruptly in an amphitheater which 

 forms a step a thousand feet in height. These gradation plains, thus 

 abruptly separated from each other, represent pauses in the eleva- 

 tions of the table-land, when the streams were depositing m^^ds and 

 gravels and forming low flats near sea level. "With the subsequent 

 rise of the land these became terraces which the streams soon began to 

 dissect and form in them deep channels and caiions. The ne^vness 

 of these canons and smaller channels is shown by the fact that they 

 are seldom more than a quarter of a mile long, although varying in 

 depth from several hundred to a few feet; and the loose gravel 

 floors, except at the margins of the terraces, have not even been 



Fig. 5. — Section of the Valley between Espeeanza and Atoyac, showing its gradient 

 to he composed of numerous abrupt steps. 



removed. These short caiions are characterized by beautiful water- 

 falls, as illustrated in Fig. &. 



The characteristics of the slopes and terraces of the valley de- 

 scending from the high plateaus have been described because they 

 illustrate evidence of great elevation, which has occurred in Mexico 

 and Central America in recent geological times, with the later move- 

 ments belonging to almost modern days. 



In place of an elevation of from six thousand to ten thousand 

 feet obtaining, as shown in many parts of the Mexican table-lands, 

 the uplift in the Tehuantepec Isthmus closing the outlet of the 

 Mexican basin into the Pacific Ocean apparently amounts to less than 



