250 The American Geologist. October, 1902. 
forces respectively should not have obtained before the ice age 
as well as afterward. The resulting loose and weathered ma- 
terials which were doubtless as voluminous as at present and 
j)ossibly even more so, readily became occluded to some extent 
in the ice mass in such a way as to become a powerful eros- 
ive agent as the ice moved. 
It is probable, also, in the light of recent investigation 
that many of the great rounded boulders that have been at- 
tributed specifically to ice action, are in reality the weathered 
products of pre-glacial time, which were simply transporteC 
by glacier ice. 
The rock substance which was first gathered in by the ice 
may have been at first largely angvdar and jagged, but became 
worn and rounded by its action on similar material, as it moved 
onward rupturing, scoring, striating, and polishing the glacier 
bed. All the work was not done upon the bed, of course, for 
the higher topographic prominences must have been affected 
along their sides by the material of the surface portions of the 
ice mass. In this case the portions that were broken from the 
protruding rocks were either borne onward to the limit 
of the glacier fiow or lodged at a more or less remote 
distance from the original rock. These transported frag- 
ments indicate, if different from the s^irrounding rocks 
of a given region, the direction of the forme'r glacial move- 
ment. It is only by noting the position of such erratics, and the 
tracing out of their paths of transport by the fflacier grooves 
and striae, often times over an extended area, that the massive- 
ness and irresistible movement of the ice sheet is understood. 
Erratic boulders of this character together with grooves 
and striae resulting from the erosive action of ice, indicate to 
an important extent the fact that certain regions of both the 
old world and the new world have been under a continental 
ice-mass, as in Greenland at the present time, and that 
there was much variation in the direction of motion, doubtless 
due to climatic conditions and underlying tocography of the 
land. 
It is evident that the grooves and striae so frequently ob- 
served upon rocks were produced in one or another of several 
ways which may here be briefly indicated. 
