296 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
description is given of the basin of the Westfield river, where the effect 
of ledges in determining the number and pattern of the terraces is very 
striking (see page 331) ; here it is briefly stated that “the materials of 
which all these terraces are formed are clay, sand, and gravel, though 
the red sandstone shows itself occasionally near the river” (20). The 
secret is told in an account of the terraces of the Deerfield: “ The river 
would encroach still further upon this hill, had it not struck a ledge of 
red sandstone, which will at least retard its lateral erosion” (19) ; and 
again, ‘**the reason why those [terraces] on Pine hill remain, I find to 
be that they rest on a protuberant mass of red sandstone. On the west 
side of the hill . . . is an ancient bed of Deerfield river . . . which was 
prevented from making any further lateral encroachments by the under- 
lying rock” (20). Yet in spite of the understanding thus shown of the 
importance of ledges in these particular instances, the generality of the 
relation of ledges and terraces is not brought forward; and the above 
instances of the local restraint exercised by ledges have never been 
quoted, so far as I can find, by any of the many readers of Hitchcock’s 
well-known essay. 
A fuller recognition of the part played by defending ledges is to be 
found in Emerson’s “ Geology of Old Hampshire County, Massachusetts.” 
It is here said that the Connecticut river in the neighborhood of Holyoke 
‘‘has now cut its bed deep in the sandstones and is thus prevented from 
oscillating ” (730). A little farther down the valley “the river early 
became entangled in rock and has cut only vertically” (733). In the 
northern part of the state “the river everywhere cut down rapidly to 
rock and has not swung widely to east and west, but has been condemned 
from the beginning to rock-cutting” (733). ‘‘ Across Chicopee there is 
a fine, low terrace bounded on the east by a high scarp of the high ter- 
race, which everywhere shows till in great force beneath the sands of 
the old lake” (730 ; see also 627, 632). It should be noted, however, 
that these passages are chiefly concerned with the occurrence of trenches, 
floored with rock and lined on both banks with ledges. The part played 
by an isolated ledge of rock or by a bank of till in preventing the 
further swinging of the river and thus defending the terraces above it is 
not brought forward. 
Ortain or Terraces IN New Encuanp. Three conclusions may 
now be stated in order that the reader may have in mind the end to 
which the preceding and following pages lead. First, a diminution of 
stream volume may have taken place during the terracing of our New 
England valleys, but it has not been essential to the production of the 
