20 The American Geologist. "^"'^'' ■^^'•'-• 
only in size, contain considerable amounts of drift, and the 
thickness of the deposit in the bottom of each basin constantly 
increases, as fresh material slides in from the top and sides. 
Obviously, these narrow basins, with steep and sometimes ver- 
tical sides are formed by the downward melting of a cylinder 
of ice beneath a cover of drift so thick that we cannot think of 
it as sensibly warmed, and still less as penetrated, by the solar 
heat, especially ais the turbid water usually completely covers 
the drift. 
These contrasts and seeming contradictions are readily ex- 
plained if we accept the standing water as an essential factor, 
and regard the detritus as secondary. Thin stones and par- 
ticles of drift absorb heat and sink into the ice ; and thicker 
stones and deposits may do the same if they chance to occupy 
water-tight depressions in the surface of the ice. A dry stone, 
if thin enough, transmits heat directly to the ice ; but with a 
submerged stone the transmission is through the medium of 
the water. Hence as soon as the thin stone or deposit becomes 
submerged the thinness ceases to be an essential factor, and it 
is on a par with the thick stone or deposit. The normal tem- 
perature of the water is, of course, that of melting ice, or four 
degrees Cent, below its temperature of maximum density. 
The water absorbs some of the solar radiation directly ; and it 
takes up promptly and completely the much larger amount of 
heat absorbed by the submerged or partially submerged de- 
tritus. As fast as the water gains in temperature it sinks to 
the bottom, displacing colder and lighter water, and expends 
its surplus heat in melting the ice. As has been noted by others, 
this principle explains the fact that the ice shores of the INIal- 
aspina lakelets are undercut below the water level, the ice, as 
(Stated by Russell, melting below the surface more rapidly than 
above, where it is exposed to the direct rays of the sun. The 
thickness of the deposit is no appreciable bar to the process, so 
long as the material is sensibly permeable ; but, on the contrary, 
it enables the water warmed by contact with its surface to sink 
promptly and quickly to the underlying ice, and with increasing 
thickness the pressure co-operates by lowering the melting 
point ; while with such torrential streams as must have been 
those in which normal eskers were formed, the conversion of 
mechanical energv into heat mav not be neglected. A bed of 
