STRUCTURAL CONTROL OF NETWORKS 



137 



for. On the one hand, the dash of the rain makes a direct attack upon 

 the uncovered fissure planes (see jflgure 12), and in the end produces the 

 well known "bad land" topography. The stages in this process may be 

 read in series of photographs taken in the "Bad Lands" area of South 

 Dakota. Mushroom Park reveals on a small scale the effect of perfect 

 rectangular jointing within a relatively thin layer. The atlas sheets rep- 

 resenting a much larger area in South Dakota all reveal a drainage system 

 controlled by joints (see figure 13). 



The Pleistocene clays of Lake Bonneville, as was long ago shown by 

 Gilbert," have a well developed simple and oriented joint system. One 



Figure 14. — Batoka Gorge of the Zambesi River 



View is taken below the Victoria Falls. The river's course is directed on a system of 

 fracture lines. The black line at bottom of gorge shows the stream width In the dry 

 season, the dotted line the water line at flood. (After Lamplugh.) 



of the diagnostic characteristics of the almost impalpable loess of the 

 same recent age is that it stands on vertical joint walls of a fracture sys- 

 tem identical with that found in the older and firmly consolidated rocks.^* 

 So far as the writer is aware, this interesting fracture system has never 

 been vouchsafed * an orientational or structural study. A remarkably 

 perfect fracture system, on which erosion has brought out with great 



" Gilbert : Monograph I, U. S. Geological Survey, pp. 211-213. 



" See Chamberlin and Salisbury : Geology, vol. 3, pp. 405-412, flgs. 523-524. 



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