alZ BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
comparatively short time swing away, after having altered its course 
more or less in a fruitless effort to wear back the obstacle. The 
ledge. thus comes to determine a cusp in the terrace front. <A salient 
of this kind may be called a defended cusp, in distinction from the 
accidental or free cusps described in the previous sections; the ter- 
race behind it cannot be destroyed by the stream. 
Following the colloquial style often adopted for field descriptions, 
a terrace of this kind is sometimes entered in my notes as a “‘ can’t-be,” 
in contrast to the low ‘‘not-yet ” terraces of Figure 22. 
It should be noted that the ledge here considered does not deter- 
mine the depth to which the river may work; the rock is exposed 
only in the river bank and enters but a little distance into the 
channel. The slope of the river and the depth to which it has cut at 
this or any other point in its course are assumed to be determined 
in all cases thus far detailed by the maintenance of an essentially 
graded channel with respect to some controlling baselevel further 
down stream ; the sea at the river mouth, a larger river into which 
the smaller stream enters, or a broad sill of rock that stretches all 
across the channel, somewhere further down the valley. 
When the withdrawing stream swings back again at a lower level, 
as in Figure 24, it cannot often under-cut and destroy all of the 
terrace on whose back border the first ledge rises, because, as has 
been noted, the slope of the ledge is seldom so steep as that of the 
terrace scarp. A second encounter with the ledge will usually be 
