DAVIS : RIVER TERRACES IN NEW ENGLAND. 297 



observed terraces. Second, the terracing rivers have slowly degraded 

 their aggraded valleys while actively swinging from side to side : deg- 

 radation probably being the result of the combined action of a slow 

 northern uplift and a gradual decrease of load, and in spite of a probable 

 decrease of volume. Third, the chance discovery of rock ledges by the 

 swinging river is the chief cause of the systematic diminution of inter- 

 scarp space and of the preservation of terraces, as seen in a typical 

 section of stepping terraces. 



A full statement of the process of terracing therefore involves a con- 

 sideration, first, of the behavior of a free-swinging, slowly degrading 

 river; and, second, of the constraint that may be imposed on such a 

 river by the accidental encounter with previously buried ledges at 

 various points in its course and at various stages in its history. This 

 anticipatory statement may aid in the understanding of the following 

 pages. 



III. The Theory of River Terraces. 



Plan of Statement. The previous paragraphs have given a gen- 

 eral consideration of several theories of river terraces, with the result of 

 deciding that one of them offers a much better explanation of observed 

 facts than the others. It is now proposed to examine the successful 

 theory with more care, first, by making a somewhat detailed study of 

 such processes of river action as are involved in the theory ; second, by 

 deducing with some minuteness the various patterns of terraces that can 

 be formed by these river processes. It will then be possible to make in 

 Part IV. a thorough test of the verity of the theory by confronting its 

 deduced consequences with the facts determined by observation. 



It should be understood that the deductive character of the succeed- 

 ing paragraphs is more apparent than real. Many features of river 

 work here presented as deductions were discovered by observation. It 

 is true that an expectation of certain occurrences had been aroused by the 

 deductive consideration of certain processes, but there has been so con- 

 tinual an interweaving of observation and theory during the growth of 

 these pages that it is now rather difficult to determine the order in 

 which the various items here recorded came to mind. It is therefore 

 chiefly for the sake of a continuous presentation of the theory of river 

 terraces that a largely deductive treatment is here adopted. When 

 the whole theory has been apprehended, it is relatively easy to test its 

 verity by observations pertinent to its different parts. 



