290 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
by a few terraces of strong scarps, or by a high terrace with one or two 
lower ones beneath it ; and it is not uncommon to find at least one side 
of a valley enclosed by a single scarp in which the whole descent is made 
at once from the highest terrace plain to the lowest. If terracing had been 
due to a general decrease in the volume of our rivers, stepping terraces 
should be much more prevalent, and broad flood plains between the high 
scarps of a siugle terrace on each side of the valley should be much more 
rare than they are ; and when the whole descent from high terrace to flood 
plain is made in a single scarp on one side of the valley, stepping ter- 
races with broad treads should be well developed on the opposite side ; 
but no such arrangement of terrace form can be said to prevail. De- 
crease of river volume must therefore be at most a subordinate cause of 
terracing, if, indeed, it is not as a rule a negligible factor in their 
production. 
This conclusion seems to have been clearly in the mind of Adams, 
state geologist of Vermont, who in 1846 wrote as follows: ‘“ The first 
stage in the process in which the terraces originated, the deposition of 
the materials, we have before referred to the older pleistocene. The 
process of denudation must have next followed, when the rivers, cutting 
down their channels through the drift barriers, lowered them gradually 
above the barriers. Flowing through the level deposits of sand, they 
must have formed serpentine channels, as rivers do now in alluvial 
plains ; consequently by increasing the convexity of the bends, and then 
cutting them off or wearing away their headlands and shifting their 
beds, they would be meanwhile removing the greater part of the ma- 
terials thus disturbed. By this process the greater portion of the orig- 
inal plain must have been carried off, and it is not necessary to suppose 
that the distance between opposite terraces is any indication of greater 
magnitude of the river, but only of its shifting its channel” (145, 
146). 
TERRACES CARVED BY STREAMS OF INCREASING Stope. When the 
basin of an aggrading river system is slightly tilted it may be expected 
that those streams whose slopes are decreasing will aggrade their valleys 
more rapidly than before (unless their point of junction with a degrading 
stream may be lowered more than their headwaters are depressed by 
tilting) ; while those whose slopes are increasing will change their action 
from aggrading te degrading. It is well known that New England has 
suffered a differential elevation in postglacial time. The postglacial 
clays of Lake Champlain and of southern Maine were deposited when the 
sea stood three hundred feet or more above its present level. The clays 
