DAVIS: RIVER TERRACES IN NEW ENGLAND. 299d 
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” (304, 
305). This seems to be an unnecessary limitation of possibilities ; for, 
as is here explained on page 305, a river that is impelled to gradual 
degradation by a slow rising or tilting of the land 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 difference 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. J[epetitions 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 
