320 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
tenth swing produces a single scarp by which the highest plain de- 
scends 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. 
Ran 
Wainer 
ll Uy 
ep” 
Errect or 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 
