AlCciiik iiJaflon of Di'ionh nx. — t j>Jiiiin. 3ol 
of small indeiitatious in the glacial boundary and conse([uont 
divergence of the latest ice-motion from its previous direction. 
If the dark bands noted in the till of Third Cliff, Peddocks 
island, and elsewhere, are marks of accumulation in successive 
years, which seems highly probable, the drumlins of Boston and 
vicinity received, at least in some instances, from one to six feet 
of till yearly deposited over their whole surface, so that the ac- 
cumitlation of these hills to their hights of 50 to 150 or 200 feet 
required only from two or three decades to a centur}' of 3'ears. 
Indeed, where they exhibit no such 1)anding, I have thought that 
sometimes their entire formation may have been more rapid, so 
that the most massiA^e drumlins, like the largest esker ridges, were 
probably deposited in so short a time that their beginning, growth, 
and completion would occupy considerably less than a man's life- 
time. The drumlins appear to have been heaped up beneath the 
ice-sheet within a few miles back from its margin, or perhaps oc- 
casionally within even less than one mile, as seems to be suggested 
by Mr. Barton's observations. Where they are scattered over any 
extensive area, probal)ly they were not all in progress of deposi- 
tion contemporaneously, but were successively accumulated as the 
ice-margin retreated. 
Quest ioii!^ fuiicrruiiii/ flic Action of th<- hi-xliict . — While some 
steps of progress seem to be gained by the foregoing observations 
and discussion, here abridged from my latest paper on this sub- 
ject three years ago, toward a knowledge of the manner and time 
of deposition of the drumlins, the questions as to how the ice- 
sheet could amass these hills, and why they are distributed in 
abundance upon some districts, but are absent or represented only 
by a few examples upon other large areas, remain to be answered. 
Their distribution, however, is to so large a degree independent 
of topographic features, and of the character of underlying rocks, 
that the explanation of their origin and grouping appears raor(v 
likely to be found in variable conditions of the glacial movements 
resulting from secular climatic changes during the final melting 
of the ice. 
ProH.VKLE .\('rC.\Il'I;.V'l"IOX (»K TlIK I )lU' M I.I NS I'HOM >]N(i I.ACI .\ I, 
Drift. 
Ohjcrtitjiix to fonncr T/iroriis. — Several theories of the way in, 
which the ice-sheet produced the drumlins have l»i'en suggested. 
