294 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
floors more and more slowly, and the tendency would then be to destroy 
all the earlier terraces by broadening the flood plain to a maximum. 
That the rate of degradation by our rivers was really slow is 
proved by the flights of stepping terraces here and there in different 
valleys; and that the normal tendency of the larger rivers is to de- 
stroy nearly all the earlier-made terraces by opening broad flood plains 
at low levels is proved by the frequent occurrence of high scarps 
descending from the highest terrace plain nearly or quite to the 
lowest. Hence it is for the preservation of high-level and intermedi- 
ate terraces rather than for their production that a more efficient 
cause than any yet discussed is to be sought. 
PRESERVATION OF TERRACES BY Rock LepcEs. What is more natural 
than that a river, swinging from side to side as it slowly degrades its 
valley floor, shall here and there be restrained on coming against a ledge 
projecting from the sloping valley wall; and that the deeper the valley 
is excavated the less breadth of free swinging can remain! This idea 
was first given explicit statement in Miller’s paper on ‘ River Terracing ; 
its Methods and Their Results,” as illustrated by observations in Scot- 
land. After a review of earlier writings this author says: 
“The modern rivers . . . have struck rock at very variable depths. 
In hundreds of cases, after winding freely about, encountering only soft 
clays and the like, and constructing terraces of various kinds, they have 
here and there become rock-bound, and prevented from pursuing their 
work of terrace-building after their former manner, as well as from de- 
stroying the terraces they had already made” (298). “When... the 
rivers commenced to work upon shallow, wide-bottomed valleys, soft and 
yielding in their nature, except where crossed by bars of rock . . . , they 
proceeded to plane far and wide, travelling from breadth to breadth to 
an extent never now equalled. With banks nowadays eight or ten 
times as high, and rock-bound at perhaps ten times as many points, it 
is no wonder that the modern rivers should seem to have ‘run in’” 
(299, 300). 
Rock ledges, however, are not here given the importance they deserve. 
The reader will not surely gain from Miller’s article a full measure of the 
value of ledges in determining the pattern of terraces, and of stepping 
terraces in particular. Hence a more detailed statement of the relation 
of ledges to terrace pattern and to terrace development seems desirable, 
especially with reference to the valleys of New England, where this ex- 
planation of terraces has not previously been applied. 
It should be further noted that certain postulates of Miller’s essay do 
