CENTRAL AMERICAN GEOLOGICAL WATER WAYS. 587 



under tlie waters of tlie Gulf of Mexico. The Teliuautepec Istlimus 

 (Fig. 7, page 588) was throughout the late Miocene and Pliocene 

 periods covered by deep water, as seen by the occurrence of such 

 fossils in the horizontal formations accumulated there, thus showing 

 that there was a strait or water way across the continental barrier, sub- 

 sequent to the great physical dislocations and changes of level occur- 

 ring in the earlier Miocene period. 



Without considering the minor oscillations of land and sea which 

 raised the coastal plains during the later Tertiary period, it has been 

 found that these plains were covered with water about its close (end 

 of the Pliocene period). Until about this time the present table- 

 lands do not appear to have been elevated. The lavas and other 

 similar rocks derived from Orizaba and sister volcanoes upon the 

 present edge of the great plateau date back only to about the close 

 of the Pliocene period, and appear to have originated with the eleva- 

 tion of the region. The excavation of the great valleys, such as 

 that described back of Vera Cruz, has been subsequent to the com- 

 mencement of this volcanic epoch, and consequently the elevation 

 of the plateau can not date back prior to about the commencement 

 of the Pleistocene period (or the beginning of the ice age). After 

 the time of the first great elevation with the formation of the 

 original valley, the region was more or less depressed, when gravels 

 and muds were accumulated upon its floor during midglacial epochs ; 

 since when the table-lands have attained their great elevation, with 

 the formation of the terrace steps already described, so recently that 

 the streams have not yet removed these loose deposits, and are only 

 in the early stages of making new caiions. 



Prom all that has now been ascertained, it appears that the great 

 elevation of Mexico and Central America was chiefly effected by 

 the Pleistocene changes of level; and from the magnitude of con- 

 tinental movements in both directions and the excavation of large 

 valleys out of very hard rocks, it would seem that this period must 

 have been one of long duration. 



The Geological AVater Way across the Tehuantepec Isth- 

 :\ros. — As has already been mentioned, the great continental table- 

 lands are here broken down for a distance of perhaps eighty miles, 

 with the resulting lower ranges for a length of twenty-five miles pene- 

 trated by numerous lower channel ways. The reduction of the width 

 of the plateau is shown in map. Fig. Y, where the shaded portion 

 represents the coastal plains setting into the highland mass, which 

 is only about twenty-five miles across. While this district was a strait 

 during the Pliocene period, in which deep water organisms were 

 living, the existing gap in the Cordilleras was being widened so as 

 in part to complete the great interruption in the American plateau. 



