1872.] ; 107 [Perry. 
rocks, is large; hence the period during which so vast a work was 
in progress,—a work requiring, according to all ordinary calculations, 
a vast length of time for its accomplishment — must have been very 
protracted. 
The material thus accumulated, as should be borne in mind, like- 
wise had a derivation, which is full of instruction. It having come, 
for the most part, from localities lying a little west of north of its 
present position,—it having generally travelled only a short distance, 
and having in all its main portions shown great constancy, in passing 
somewhat obliquely across meridional valleys and hills,—it is difficult 
to see how it could have been in any wise so transported by water, 
as uniformly to exhibit these characteristics. Meanwhile, as to 
origin and distance of removal, as to direction and mode of trans- 
portation, it has observed all the conditions, and shows every mark, 
which we may conceive it would have observed and be likely to 
show, had it been the result of glacier-action. 
While such seems to be the character of the largest portion of the 
typical drift among the hills of New England, there are occasional 
exceptions in the form of erratics. These are far travelled, and com- 
posed of tough material; they are hard-heads indeed, in the literal 
sense of the expression; thus they were likely to be preserved, in 
spite of all the wear and tear to which they must have been exposed 
in their removal, in their protracted journey, and during the long 
subsequent lapse of time. That other rock-masses which set out 
with them, were ground to powder, and thus unable to tell precisely 
the same story, is no doubt true. \ The fact that the rounded blocks 
which remain travelled so far, and only just survived the vicissitudes 
to which they were exposed, involves a significant piece of evidence. 
Bearing in mind that the continental ice-sheet probably had only an 
infinitesimal motion, in comparison with that of the local glaciers 
that now descend mountain valleys,—that the portion which bore 
the far-travelled boulders, was doubtless equally slow; and that 
these rocks were, perhaps, transported the greater part of the way 
during the Middle Glacial times,— we find ourselves possessed of a 
new element, indicating that the winter of the ages must have been 
of long duration. 
The position of the materials of the drift-sheet is also character- 
istic. Matter having been derived as supposed, it having been 
forced along beneath the mass of ice, and to a limited extent in front 
of it, in some such ways as have been indicated, we readily see that, 
