130 PHYSICAL GEOLOGY 



5. Flint nodules are of common occurrence in limestone (p. 121). 

 Explain the fact that in certain limestone regions the stream beds 

 contain few limestone bowlders, but many of flint. 



6. Why does residual mantle rock in many cases merge gradually 

 into the firm rock beneath? (See Fig. 214.) 



7. Describe and explain what you see in Figure 121. 



FIG. 121. Granite rocks. Laramie Hills, Wyo. 

 TRANSPORTATION BY STREAMS 



Getting a load. Streams roll and drag material along 

 their beds, and carry it in mechanical suspension and in 

 solution. That which is moved mechanically is obtained 

 in a variety of ways. Streams wear material from their 

 beds and banks, and remove that which is already loose. 

 Sediment is brought in by tributaries. Material loosened 

 by weathering on the tributary slopes is delivered to the 

 stream by gravity or by rain wash. A certain amount of 

 fine material is brought by the wind. Most of the material 

 which is carried in solution is contributed by issuing ground 

 waters; a small part is furnished by the unorganized run- 

 off, and another minor portion is dissolved by streams from 

 the rocks over which they flow. 



How the load is carried. A stream pushes sediment along 

 its bottom by the direct impact of the current, and also 

 rolls and drags it by the friction of the bottom water, some- 



