DAVIS: RIVER TERRACES IN NEW ENGLAND. 295 



not command entire assent. He states that " it is not allowable to have 

 recourse to coast elevation, or climatic changes, or periodicity of any kind, 

 without first proving that the terraces range in opposite pairs " (30-t, 

 305). This seems to be an unnecessary limitation of possibilities ; for, 

 us is here explained on page 305, a river that is impelled to gradual ' 

 degradation by a slow rising or tilting of the hind may produce unpaired 

 terraces as it wanders to and fro across its valley floor. On another 

 page Miller concludes that rivers " cannot but concentrate their channels 

 as they excavate them, unless the amount of planation is out of all 

 proportion to the rate of deepening " (300), and seems to imply in this 

 statement that a large ratio of lateral erosion to degradation, such as is 

 here assumed for our New England rivers, and further consid- 

 ered in later sections, is an improbable ratio. To this it may be an- 

 swered that the occurrence of stepping terraces at one and another point 

 in our larger valleys certainly justifies the assumed ratio by showing 

 that lateral swinging should be measured in hundreds or thousands of 

 feet at many successive stages of degradation, while the total degradation 

 is usually to be measured in tens of feet and seldom exceeds one or two 

 hundred feet. A possible reason for the diffei'ence of values given to 

 this ratio may be that Miller's studies were directed to the moderate-sized 

 rivers of Scotland, while the best terraced valleys in New England are 

 those of large rivers like the Connecticut and the Merrimac and 

 their stronger branches ; and, as has been already pointed out, a large 

 river may swing actively during an uplift that gives a small river no 

 time for anything but down-cutting. These two items are, however, of 

 secondary importance in Miller's theory compared to rock ledges. 



In reviewing various other essays of earlier dates several suggestive 

 passages have been found, hinting at the importance of rock ledges. 

 Adams makes the following statement : " If a terrace has been formed 

 before the complete removal of the obstructions in the channel [the con- 

 text shows that these obstructions are ' solid rock '], the same process 

 must have been repeated within the new and narrower level of interval. 

 We should thus have a second terrace. Repetitions of the process in 

 cases where the obstructions were not entirely removed would occasion 

 agreater number of terraces " (146). Something more explicit is found in 

 Edward Hitchcock's "Surface Geology." In describing a middle section 

 of the Connecticut valley, where the terraces became famous from the 

 writings of this author, it is said that " the rock often projects through 

 the terraces" (18), but the service of the rock in protecting the over- 

 lying terrace from being cut back is not announced. Farther on a 



VOL. XXXVIII. — NO. 7. 2 



