REPEATING PATTERNS IN THE EARTH RELIEF 225 



\ 



the Abisko Canon of northern Lapland (Fig. 240). We are later 

 to learn that another great transporting agent, the water wave, 

 makes a selective attack upon the litho- \ /, 



sphere along the fractures of the joint 

 system (Fig. 250, p. 233 and Fig. 254, 

 p. 235). 



Where the scale of the example is large, 

 as in the cases which have been above 

 cited, the actual position and directions 

 of the joint wall are easily compared with 

 the near-by elements of the river's course, 

 so that the connection of the drainage 

 lines with the underlying structure is at 

 once apparent. In many examples where 

 the scale is small, the evidence for the con- 

 trolling influence of the rock structure in 

 determining the courses of streams may 

 be found in the peculiar character of the drainage 



FIG. 240. Map of the 

 joint-controlled Abisko 

 Canon in northern Lap- 

 land (after Otto Sjogren). 



plan. To 



illustrate : the course of the Zambesi River, within the gorge below 

 the famous Victoria Falls, not only makes repeated turnings at a 

 right angle, but its tributary streams, instead of making the usual 



sharp angle where they join the 

 main stream, also affect the right 

 angle in their junctions (Fig. 241). 

 The repeating pattern in drainage 

 networks. It is a characteristic of 

 the joint system that the fractures 

 within each series are spaced with 

 approximation to uniformity. If 

 the plan of a drainage system has 

 been regulated in conformity with 

 the architecture of the underlying 

 rock basement, the same repeating 

 rectangles of the master joints may 



be expected to appear in the lines of drainage the so-called 

 drainage network. 



Such rectangular patterns do very generally appear in the 

 drainage network, though they are often masked upon modern 

 maps by what, to the geologist, seems impertinent intrusion of the 

 Q 



FIG. 241. Map of the gorge of the 

 Zambesi River below the Victoria 

 Falls (after Lamplugh). 



