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bulletin: museum of comparative zoology. 



tenth swing produces a single scarp by which the highest plain de- 

 Bcends to the river level. Then the eleventh and twelfth swings are 

 held off from the high scarp by a lower ledge on whose slope two 

 low-scarped terraces are carved. It may therefore be concluded that low 

 undefended high-level terraces of early swings are most likely to be pre- 

 served back of defended cusps of later swings ; that the undefended 

 terraces of early swings will probably be swept away in the production 

 of a single high-scarped terrace wherever broad swinging at low levels 

 is not prevented ; and that when high scarps occur in a flight of step- 

 ping terraces they are more likely to be found at or near the top than 

 near the bottom of the flight. 



Effect of Rock Barriers. Superposition upon strong rock barriers 

 has already been considered on pages 292 and 309, in so far as it deter- 

 mines the separation of a valley into several compartments, in each of 

 which the flood plain is thenceforward graded with respect to the 

 next down-valley barrier. This is a very familiar condition in New 

 England, as the water-power in the falls or rapids on the down-valley 

 side of the barriers has repeatedly determined the location of our manu- 

 facturing villages and cities. In the present section, brief consideration 

 is given to the effect of rock barriers in producing a fixed node, as 

 Emerson has called it (736), in a stream that elsewhere vibrates freely 

 as it meanders and swings on its flood plain. It is, however, not yet 



