150 GEOLOGY. 



In the history of these rivers, however, a factor is involved which has 

 not yet been considered, and these streams will be referred to later. 



As base-level is approached, the outcrops of hard rock are brought 

 low. When they have been reduced to the level of their surroundings, 

 the streams may flow without regard to the resistance of the rock 

 beneath, for downward cutting has ceased. As this stage of erosion is 

 approached, a readjustment of the drainage may take place, and the 

 waters which had taken long and circuitous courses to avoid hard rock, 

 may change their courses to more direct ones (compare Figs. 130 and 

 135). Adjustment is, therefore, a relative term, and streams which 

 are adjusted at one stage of erosion, are not necessarily adjusted at 

 another. 



It sometimes happens that rocks of unequal resistance are covered 

 by beds of uniform hardness. A consequent stream developed on the 

 latter may find itself out of structural adjustment when it has cut 

 its channel down to the level of the heterogeneous beds below. Such 

 a stream is said to be superiinposed (Fig. 136) on the underlying struc- 

 ture. Structural adjustment is likely to follow. 



INFLUENCE OF JOINTS AND FOLDS. 



Joints. — Various structural features of rock other than hardness 

 influence its erosion. Apart from the stratification planes, most rock 

 formations are affected by joints or fissures. The joints are often, 

 but not always, nearly vertical. Two sets are generally present, and 

 sometimes more. If but two, they usually meet at a large angle; if 

 more than two, two are likely to be nearly perpendicular to each other, 

 while the third and fourth sets have such directions as to cut the others 

 at large angles. These joints allow the ingress of water, roots, etc., 

 which help to weather and disrupt rocks. Occasionally there is notable 

 sag of the beds of rock along joint planes, but this effect is usually 

 superficial only (Fig. 137). Where the jointage planes are frequent 

 and open, the columns bounded by them sometimes topple over on 

 cliff faces, either by undercutting, or by the wedge-work of roots or ice. 



The effect of joints on erosion may often be seen along a stream 

 which flows in a rock gorge. In such situations, the outlines of the 

 banks are sometimes angular, and sometimes crenate (Fig. 138), the 



