;^2 The American Geologist. J"^^' i^^^- 
discussion which follows the tunnels arc assumed and not 
accounted for. 
Topographic Relations of Eskcrs. — The main points un- 
der this head have been duly considered ; and it remains sim- 
pl_\- to note once more that they are, practically without excep- 
tion, more accordant with the superglacial than the subglacial 
hypothesis. The very usual haphazard relation of an esker 
to the contours of the bed-rock and till are seen to be entirely 
normal for a superglacial deposit ; while from the 'subglacial 
point of view they are a perpetual enigma. Besides the general 
indifference of their trends to the modern drainage, which 
has, perhaps, been sufficiently considered, we have the fact 
that they are not, as a rule, distinctly contrasted in size or in 
the coarseness of the gravel on northern and southern slopes. 
The subglacial stream was comparable with the flow of water 
in a pipe; and the velocity for any given head was inversely 
proportional to the diameter of the tunnel, and independent 
of the local gradient, the velocity being the same on northern 
or ascending as on southern or descending slopes. But, ob- 
viously, the tendency to aggrading of the stream bed is far 
greater on northern than on southern slopes, in spite of the 
uniform velocity ; and it is also obvious that the esker of sub- 
glacial origin should be stronger, more perfectly aggraded, 
and composed of much coarser material on the up than on the 
down slopes. This contrast may. perhaps, be noted occasion- 
ally, but it is by no means so marked or general as the hypoth- 
esis requires. 
Eskers belong chiefiy to the moderately dissected pene- 
plain tracts and are not especially characteristic of deep moun- 
tain valleys where, unquestionably, the conditions were most 
favorable to crevassing and a concentration of the glacial 
drainage beneath the ice. Again, the usual lateral position of 
valley eskers is entirely normal for the superglacial and inex- 
plicable for- the subglacial hypothesis. To cite a single in- 
stance, in the village of Bridgewater, Nova Scotia, on the 
southwest side of the La Have river, the valley of which is 
here some three hundred feet in depth, a well formed esker 
trends approximately parallel with the valley and at an ele- 
vation of about one hundred feet above the rivy^r- That a sub- 
glacial stream of water could have hung on this steep slope 
