Page:Popular Science Monthly Volume 53.djvu/607

Rh under the waters of the Gulf of Mexico. The Tehuantepec Isthmus (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, subsequent to the great physical dislocations and changes of level occurring 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 tablelands 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 elevation of the region. The excavation of the great valleys, such as that described back of Vera Cruz, has been subsequent to the commencement 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 cañons.

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 continental 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.

—As has already been mentioned, the great continental tablelands are here broken down for a distance of perhaps eighty miles, with the resulting lower ranges for a length of twenty-five miles penetrated by numerous lower channel ways. The reduction of the width of the plateau is shown in map. Fig. 7, 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.