THE WORK OF STREAMS 



103 



for considerable stretches, follow lines of dislocations. (4) Where the 

 rock over which a stream flows is strongly jointed (Fig. 85), the 

 joints are sometimes followed to some extent by the smaller tribu- 

 taries. Larger streams, however, are less affected, usually showing 

 little evidence of this influence. (5) When streams flow through 

 structural valleys (p. 100), their direction is necessarily predeter- 

 mined. (6) In a region underlain by horizontally bedded rock, the 



Fig. 85. — Fi 



Creek, South Dakota. Showing the effect of jointing on the 

 course of a stream. 



valleys extend in many directions without systematic arrangement 

 and are described as dendritic (treelike). Such a river system is in 

 striking contrast to one developed in a region of tilted strata in which 

 the beds vary in their resistance to erosion. In such a region the 

 tributaries have a trellised appearance (Fig. 92, p. 107). 



Basins and Divides. — All the land surface which is drained by a 

 river and its tributaries is called its hydro graphical or drainage 

 basin, and the boundary between two river basins is termed 

 the divide, since the water falling on it is divided, part flowing into 

 one river system and part into the other. A part of the Great Conti- 



