260 



PHYSICAL GEOLOGY 



ice and of other brittle substances were twisted by applying force at 

 the two ends, produced cracks at right angles to each other, running 

 diagonally from the corners. 



The formation of columns in fine-grained, igneous rock is probably 

 due to cooling and contraction, as has been explained (see also p. 333). 



Effect of Joints on Topography. — The influence of joints in de- 

 termining the courses of streams is especially noticeable in small 

 streams. The brooks that flow into the lakes of central. New York, 

 for example, have their directions determined to an important degree 

 by the systems of joints into which the strata are broken. The 

 course of the Zambezi River in South Africa below Victoria Falls is a 



Fig. 251. — A shore whose configuration has been greatly influenced by jointing. 

 Holsteenborg, West Greenland. (After Hobbs.) 



remarkable example of a large river which follows joints for a con- 

 siderable distance. For many miles after it plunges over the fall 

 the river is confined in a narrow gorge whose direction is a series of 

 zigzags, this angular course being determined by the joints of the 

 lava plateau in which the gorge is cut. The contours of coasts 

 sometimes show the effect of jointing by the existence of angular bays 

 and promontories (Fig. 251) where the rock is unequally jointed, 

 permitting the waves to work faster in some portions than in others 

 (p. 207). Joints also allow of more rapid work by weathering (p. 29), 

 more rapid broadening of stream valleys (p. 88), the circulation of 

 ground water, and the deepening of glacial valleys by plucking 

 (p. 157). In limestone regions joints are often widened by solution 

 and therefore have a marked effect on the topography of such districts. 



