The Regions of Transition between Highlands and Lowlands; the River Systems of the Globe.
Between the two great and most sharply-marked physical features—the high plateaus and mountains and the lands of very little elevation—there are regions of transition very numerous and exceedingly varied.
The conception of highlands and of lowlands having a certain, constant, and absolute value, and it being immaterial whether the elevation be specially marked or not, provided it be uniform, the regions of transition find their most marked characteristics in their want of constancy, in their very change, and the rate at which the grade ascends from a low to a high elevation, or falls from a high to a low one. Their real value lies in the mutual compensation of highlands and lowlands, which is effected through the mediation of a third physical feature or system, which has received the name Lands of Gradation, or Terrace Lands, and which, by their gradual rise from the sea level, serve as the means of transition from the lowest lowlands to the loftiest plateaus and mountains.