IN THE WHITE MOUNTAINS. 558 
raines, consisting of loose materials collected along the sides 
and at the terminus of a glacier, always indicate, and, where 
undisturbed, actually define the margins of a moving mass 
of ice; whereas the so-called median moraines PIGH along 
the line of junction of the glaciers are carried upon the back 
or upper surface of the ice, and always consist of angular 
materials, the shape and arrangement of which are deter- 
mined by their mode of accumulation. Just as among the 
glaciers of the present day we discriminate between ground 
moraines, lateral, frontal and median moraines, so must we 
also distinguish between the same phenomena in past times. 
The glacial period had also its ground moraines, its lateral, 
its frontal and its median moraines, its erratics and perched 
boulders. But the huge ground moraine of the earlier ice 
time stretched continuously, like the ice sheet under which 
it was formed, over the whole country — from the Aretics to 
the Southern States, and from the Atlantic to the Rocky 
Mountains. I do not speak of the western slope of the Con- 
tinent, because I have not examined it personally. The 
great angular erraties of that period were scattered irregu- 
larly over the country, as the few large boulders are icutéétud: 
on the upper surface of a glacier now. It is the contact of 
the more limited phenomena of the local glaciers which suc- 
ceeded this all embracing winter (their lateral, frontal, me- 
dian and limited ground moraines and their erratics), with 
the more wide-spread and general features of the drift that I 
have been able to trace in the White Mountains this summer. 
The limits of this paper will not allow me to do more than 
record the general facts, but I hope to give them hereafter 
more in detail and with fuller illustrations. The most diffi- 
cult part of the investigation is the tracing of the erraties to 
their origin; it is far more intricate than the identification of 
the origin of ordinary drift, or of continuous moraines, be- 
cause the solution of the problem can only be reached under 
favorable circumstances where boulders of the same kind of 
rock can be followed from distance to distance, to the ledge 
AMER. NATURALIST, VOL. IV. 70 
