262 PROCEEDINGS : BOSTON SOCIETY NATURAL HISTORY. 
near vicinity of the ledges. The significance of this in relation to 
questions concerning the nature and origin of ground moraines is 
of no particular importance, for the resulting intermixture is readily 
obtained whether the ground moraine is composed of material torn 
from the ledges and dragged along beneath the ice, or whether it 
is composed of englacial material set free by basal melting. 
Regarding the dying out with distance little more than that 
given on p. 257-258 need be said. As would be expected from the 
fragile nature of .a large percentage of the fragments torn from 
the ledges, it would at first be extremely rapid, but would become 
less and less abrupt, as the distance increased, until it finally disap¬ 
peared. If we should make a plot with the number of boulders 
as ordinates and the distances as abscissae, the resulting curve 
would have the form of a parabola. 
The rapidity of the dying out varies considerably, according to 
the nature of the till. Dividing the till deposits into those once 
forming a part of the ground moraine and those derived from 
englacial material, we should, theoretically, find the decrease in 
the number of red fragments to be far more gradual in the latter 
deposits than in the former. Various deposits of till were studied 
with this view in mind, and the observations all point strongly to 
such a relation though the difficulty of readily distinguishing the 
two classes of till may, in a few cases, have led to erroneous con¬ 
clusions. 
Distribution in modified drift .— The differences in the rates 
of dving out of the boulders in the till and in the modified drift 
are still more marked than in the case of the two varieties of till, 
there being, apparently, several times as many pebbles in the 
deposits of modified drift as in the deposits of till at equal distances 
from the source. There seems to be no doubt that the sand plains 
which furnished most of the sections, from the study of which the 
above proportion was obtained, were formed by streams either of 
superglacial or englacial character. The material was, therefore, 
of englacial derivation which readily accounts for its preservation 
at greater distances than in the till. 
An even more significant feature of the distribution of pebbles 
in the modified drift is found in the fact that in deposits of this 
nature near the ledges the pebbles are no more abundant, and in 
some cases apparently less abundant, than in the stratified deposits 
at a distance of five or ten miles. This is evidently due in part to 
