'•■)52 TIk' American (JciiJogixt. Dt'ccmbur, is92 
Tlu- eiiiliest was ))y Slialcr in 1870, who supposed these hills in 
the vicinity of Boston to l)e remnants spared from the tiuviatile 
and tidal erosion of a once eontinnoiis sheet of drift, which had 
l)een contained in ai>hicier that descended the Charles river valley. 
His later view is similar, but attributes the very thick drift sheet 
of this hypothesis to deposition during the earlier of two epochs 
of glaciation, and its erosion partly to sea and river action during 
an interglaciul epcjch, but mainly, for the peculiar sculpture of 
the drnmlins, to excavation and removal of the drift from all the 
intervening areas by the later glaciation. To accord with this 
view, however, the terminal moraines of the later ice-sheet must 
vastly exceed their very moderate observed volume. Another 
objection, pointed out by Salisbury, is that the (b-umlins appear 
to be composed wholly of the newer drift. 
Hitchcock and Wright have thought the drnmlins to be perhaps 
the material of terminal moraines swept over and massed in these 
peculiar forms by subsequent farther advances of the ice-sheet. 
If this view were true, the lill of the drnmlins could not have its 
nearly uniform charactei', Init would contain here and there re- 
markal)le aggregations of boulders, and frequent irregular en- 
closures of sand and gravel would 1)e found, representing the 
kame deposits and lenticular l)eds of modified drift which so 
commonl}' make up considerable parts of the terminal moraines. 
Salisbury remarks that neither the distribution nor the composi- 
tion of the drnmlins seems to favor this hvpothesis, and he there- 
fore believes that they were built up Ijeneath the ice, not being 
fashioned from hills ovei-ridden by it. 
Mr. Clarenoe King and Prof. J. D. Dana have conjectured that 
the drnmlins, at least in some cases, were made by superglacial 
streams, charged with drift, pouring through crevasses or a moulin 
to the land surface, there depositing their drift, which afterward 
by the onflow of the ice would be subjected to its pressure and 
sculpturing. This explanation lies under similar objections with 
the last. 
Kinahan and Close in Ir^'huid. Prof. James Geikie in Scotland, 
and Davis and Salisbury in this country, look on the drnmlins as 
analogous to the sand liars of streams. Professor Davis writes: 
In view of the irregularity of the surface on which the ice-sheet 
moved, and of the greater w^eakness of some rocks than others, we 
must sii|)]>()se an irrcgiihir v(H<)<'itv in the motion of the ice and an 
