232 C. R. KEYES — CRUSTAL ADJUSTMENT IN MISSISSIPPI VALLEY. 



subsequent considerations will show. The phenomena here recorded 

 apply more especially to the western interior coal basin occupied in 

 part by Iowa and Missouri, as in this district most of the observations 

 were made. 



■ Carboniferous Basin of the Mississippi Valley. 



Extent. — The broad undulatory plain which occupies the central por- 

 tion of the American continent, stretching out from the base of the 

 Appalachians to the foot-hills of the Rocky mountains, makes up the 

 principal portion of what is known as the Continental Interior region. 

 It spreads out in one direction for more than nine hundred miles and 

 in another over twelve hundred. Its superficial contents are upward 

 of one million square miles, or more than one-third of the entire areal 

 mileage of the United States. This vast expanse of country, whose sur- 

 face is unbroken by mountains and whose borders are untouched by the 

 w^aters of the sea, has been aptly designated a great basin. The Great 

 Mississippi basin it is called, from the majestic river, the " Father of 

 Waters," which flows centrally through it. 



The region may be properly regarded as a wide stretch of lowland 

 sloping gently in all directions from the margins toward the center and 

 southward. The " Great iPlains " form the western portion of the region ; 

 the rolling prairies of the " Upper Mississippi " the median part ; the 

 fertile valley of the Ohio and the Cumberland plateau the eastern sec- 

 tion. No marked contrasts of altitude break the surface relief of the 

 Mississippi basin. The lowest point, in the south-central part, is about 

 four hundred feet above tide level ; the highest places are on the northern 

 and western margins, where the mean elevation is not far from two 

 thousand feet. 



The contrasts of relief to be noted in considering the Interior basin 

 are not those between difl'erent parts of the plain itself, but those between 

 the basin as a whole and the region immediately around it. Beyond 

 the boundaries in nearly every direction a mountainous physiognomy 

 is presented. The Appalachians on the east and southeast, the Rocky 

 mountains on the west, the highlands of the Great lakes region north- 

 ward, all stand out sharply against the country they surround. They 

 all tell of powerful dynamic action which has been at work elevating- 

 broad stretches of territory ; of continental movements which have oper- 

 ated on a grand scale. 



On the whole, erosive agencies have not acted vigorously since the de- 

 posits of the Mississippi valley were originally laid down in the old Car- 

 boniferous seas. Through most of the long period, from the time when 

 the beds were first raised above the level of the waters of the great 



