THE GREAT PLAINS OF THE UNITED STATES 275 



of land was tilted upward when the crust of the earth was 

 folded upward along the great continental divide. 



The elevations are either flat-topped hills, the strata of 

 which are slightly inclined and correspond in position to 

 those found in the plain beneath, or they are masses of ig- 

 neous material which appear to have been thrust up through 

 the rock surrounding them. In the former case the ele- 

 vations are simply remnants of the layers of rocks which 

 once extended over the country, but which have now been 

 eroded away over the larger part of it; in the latter case 

 they are the igneous masses which have withstood erosion. 



SUMMARY 



Almost all scientists agree that the matter of the earth 

 was once in a nebulous state. From this it came into an 

 exceedingly hot liquid condition and then into a solid state. 

 The interior of the earth is still hot, but the outside part, 

 or crust, is cold. As the interior of the earth is still cooling 

 and contracting, the crust must fold in order still to rest 

 on the shrinking interior. Thus the surface of the earth 

 has been slowly changing through the ages, and it continues 

 to be modified. For example, the sea coast is not stable 

 but is subject to upward and downward movements. The 

 surface of the land is rough and irregular and different land 

 areas vary greatly in composition, in the warping and fold- 

 ing of rock layers and in the positions of these layers. The 

 rocks of the earth's crust are divided into three groups : 

 igneous, which have solidified from a melted condition; 

 sedimentary, which are made by deposition in water; and 

 metamorphic, which are forms of igneous or sedimentary 

 rocks that have been modified by natural forces. 



