512 



THE THEORY OF EVOLIFTION 



A 



^ )M l -a U^ A -':^' - ^^ !^^. ^^^r4 





Fig. 274. — How the geologist interprets facts that tell of the evolution of 



the earth's surface. 



. A, section of an exposure similar to that shown as Fig. 273, in which the underlying 

 solid rock has weathered into loose mantle rock and overlying soil. B, section showing a 

 former land surface (c-d) upon which boulders, gravel, and sand have been depo-sited by 

 flowing water, as indicated by the horizontal layers. C, section similar to B showing a 

 former land surface (c-d) upon which are glacial deposits as indicated by the irregular 

 distribution of boulders, gravel, and sand. D, section of a mountainous region, showing 

 how the folding of the rock layers and the former land surface (c-d) can be reconstructed 

 by study of the angles at which the rock outcrops occur on the existing land surface (a-6) 

 which has resulted from the erosion, a-b, existing land surfaces; c-d, former land surfaces 

 ((■/■. Fig. 205). (A to C, after Branson, "Geology of Missouri," University of Missouri 

 Bulletin, Vol, 19.) 



