1S92.] 13 
[Uphar 
tion in sinking as the underlying ice melted. Such fractures 
extending deeply into this hill were also found in the construction 
of the reservoir on its top, which gave much trouble by leaking, 
until the bottom was made impervious with cement. The till 
where not so fractured is water-tight, and numerous reservoirs on 
other drumlins near Boston have been free from this difficulty. 
The narrowness of Powderhorn Hill, in proportion to its length, 
probably caused it to sink more unequally than most of the drum- 
lins in this district. 
On some other areas, and perhaps more commonly, di-umlins 
may have been formed from the englacial stratum of drift dui'ing 
a time of re-advance of the ice-sheet, carrying the drift forward 
so that it would be accumulated on a land surface. This appears 
to have been the case in central Xew York, where Prof. W. M. 
Davis finds that the sections of drumlins frequentl}^ show strati- 
fied gravel and sand undei-lying the till, and that often the rela- 
tionship of these formations is such as to prove that the stratified 
beds were somewhat eroded before the deposition of the till. But 
the absence of such sections in Massachusetts and generally in 
New England makes it probable, as stated, that here the reces- 
sion of the ice was continued, though with a much diminished 
rate, while the drumlins were being amassed. 
4. REVIEW OF OBJECTIONS TO THIS EXPLANATION. 
At first sight, this explanation of the accumulation of the drum- 
lins appears to be opposed by two cons})icuous objections, which 
must be answered. Tiie first is the local derivation of much of 
their material. Where the peculiarities and restricted limits of 
the adjacent lock formations on the north i)ermit an ajijiroxiinate 
determination of the distances of transportation of the drift form- 
ing the drumlins, it is found that a large part, sometimes more 
than half, has been carried only a few miles. It seems suri)rising 
that local material should constitute so important an element of 
the drift contained within the ice at considerable heights, until 
we consider how fast it would be uplifted by even a very slight 
upward inclination of the basal current of the ice. If the diift 
eroded from any place was carried up with an average ascent of 
only one degree, it would rise within one mile to an altitude of 92 
feet above the ground, and within two to tJjree miles would be 
