582 Surface Geology of Eastern Massachusetts.  [October, 
A comparison of the lacustrine depressions of this region of 
excessive drift with those of Worcester County, on the one 
hand, and of Essex, Middlesex, Norfolk, Bristol, and Northern 
Plymouth counties, on the other, shows that the value of the 
mean elongation in the former district, 2.7, is intermediate be- 
tween the values of the same property in the two latter, 3.15 
and 2.3, respectively. And thus we are brought to the follow- 
ing general conclusion: where the drift is so deep that the forms 
of the lake basins have no necessary relation with the subjacent 
rocks the mean elongation of the depressions is greater than in 
districts where, the detrital sheet being thinner and less univer- 
sal, the basins are partially rock-bordered, — when the progress 
of the ice-cap was transverse to the general strike or structure 
lines of the rocks, — and less when this movement coincided 
with the strike. 
The reliefs of this region are, for the most part, of very mod- 
erate altitude ; and, in consequence of the sharper contrast be- 
tween land and water than between hill and valley, they are 
seldom represented on maps with even an approximation to the 
accuracy characterizing the delineation of water-bordered con- 
tours. Hence it were futile to attempt to discuss our hills and 
ridges in the same manner as the lake basins and river valleys. 
Fortunately, however, the general facts are so plain that they 
do not require this for their elucidation. It is in the experience 
of most observers in this region, that the drift hills have usually 
a lenticular outline, are more or less ridge-like, and that both 
hills and ridges coincide in trend with the direction of glacial 
movement. Those remarkable drift ridges in Essex County, de- 
scribed by Mr. G. F. Wright, and extending with a nearly rec- 
tilinear course from beyond the New Hampshire boundary to 
Massachusetts Bay, exemplify in a striking manner the form, 
trend, and general independence of geology characterizing roa 
drift topography. Elevations composed mainly of rock ™ sit, 
on the contrary, express in their forms and trends the leading 
geologic structure lines of the region, but do not admit of re 
relation with the course of glacial action. The Waar 
range of highlands, the parallel range forming the eastern ay 
of the Nashua valley, the somewhat irregular belt of hills ex 
tending from Cape Ann to Beverly, the well-known range ke 
ing with a bold front from Swampscott to Waltham, et i 
Blue Hill range in Milton and Quincy are good examp ta 
the more prominent and general of our rock reliefs. 
