404 FIRST TEAR SCIENCE 



called the Great Plains. This region consists of irregular 

 intrenched valleys 50 to 100 feet deep. Sometimes there 

 are hills and mountains, but viewed from an eminence the 

 country appears flat. 



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- 



A HIGH, DRY PLAIN. 



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. 

 The Great Plains may thus be considered as an example 

 of a plain of erosion. 



Here, as in the prairie region, trees are wanting, but 

 their absence is due rather to the lack of the necessary 

 rainfall than to the reasons assigned for the former region. 

 Although formerly considered almost a desert on account 

 of its small rainfall, this region now supports vast herds 



