THE EIVER GORGE. 55 



suiting from the vast erosions that have taken place along its 

 western border. In central Tennessee, indeed, this erosive 

 process has excavated a basin a hundred miles in diameter, 

 bounded on all sides by the ragged edges of the formations 

 which were left. 



So this completed work of erosive powers which have re- 

 tired from action is commemorated in many a monument-like 

 outlier in Wisconsin and Minnesota. A great formation 

 which once overspread many a township has all been carried 

 away, save here and there an isolated remnant which lies like 

 an island in the midst of geology of a different character. It 

 is the Potsdam Sandstone which has been thus eroded ; but I 

 wide areas still remain, and underlie portions of those states. 

 Similar are the columns in Monument Park, and the ruins in 

 the " Garden of the gods." Like the great basin of central 

 Tennessee are many of the excavations in the Bad Lands of 

 the Upper Missouri and in New Mexico. We shall have 

 other occasion to talk about these ; for they are burial places 

 of the brute populations which held possession of America 

 before the advent of man. 



These two great processes, erosion and sedimentation, / 

 must be vividly appreciated. The whole history of the visi-/ 

 ble land has consisted chiefly of up-building and destruction, 

 rebuilding and disintegration, by the action of forces which 

 have left gigantic monuments of their former power, and 

 are even in our times, working on a scale large enough 

 to illustrate to us how the foundations of the land were laid, 

 and how the face of the earth has been carved into the fashion 

 it presents to our interested eyes. 



In another walk we must follow the sediments, under the 

 sea, and try to learn what goes on in the mysterious abysses 

 through which no highway has been opened, 



