294 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



floors more arid more slowly, aud 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 Ledges. 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 uever 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 



