STRUCTURAL CONTROL OF NETWORKS 



139 



within the rock basement is obvionsly less than it is where the surface is 

 unprotected by vegetation, yet accurate maps must show in how far a 

 direction control exists. That the influence of frost work will be lessened 

 is certain, but other processes are also subject to control (see plate 11). 

 Experience shows that areas underlain by limestone are especially re- 

 sponsive to directing influences during denudation because of the solvent 

 property of the water which percolates through them on fracture planes. 

 This is illustrated by plate 11, figure 1, which shows joint planes widened 

 by solution within compact crystalline limestone. How the upper layers 

 of a relatively flaggy type of limestone come to sag over joints, and thus 

 start the course of waterways, is shown in figure 15. 



Figure 16. — Map of the Vicinity of the Moldefjord in western Norway 



Showing how one relief pattern (about the Langfjord) is merged in another (about the 



Bikesdalsviken) 



A study of accurate hydrographic maps, as already stated, may be 

 relied upon to show in how far fracture control is exercised in those dis- 

 tricts of lower latitude where vegetable cover and other modes of weather- 

 ing are the rule. Ignoring, then, for the moment the theoretical side of 

 the problem, and attempting only to observe the evidences of systematic 

 control of drainage in accordance with any repeating pattern, it may be 

 said that many excellent observers have supplied striking examples of 

 such control and from widely separated districts. 



The best known examples are those given by Daubree in his monu- 

 mental work on experimental geology, in which regularly oriented net- 



xr — Bull. Geol. Soc. Am., Vol. 22, 1910 



