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Association of American bo 175 
Prof. Henry D. Rogers said there was need of much caution 
in the use of the term bowlder, as regards the size of the mass to 
which it should be restricted ; he was inclined to give the term 
much latitude. Thus he conceived that a current of drift com- 
ing from the north and meeting the terraces of Pennsylvania, 
would there be arrested and deposit its larger masses—an 
from stage to stage, until the onward current would carry for- 
ward only the smallest sand; in this way, we may find among 
the drift of the south, all the materials derived from the northern 
rocks. 
He concluded that all the materials of a current of drift, find 
their resting place in accordance with gravity. 
Prof. Mather doubted whether the large bowlders found in 
Long Island, resting on beds of sand or fine gravel, could be 
thus accounted for, because a current of sufficient force to move 
such large masses would have carried away the sand. 
Prof. Rogers replied, that diluvial action could not be restricted 
to a single epoch. ; : 
We must find in secular and periodical elevation, the cause of 
the translation of the beds of infusorial earth recently found in 
the tertiary of Virginia, which are there covered by the quiet 
strata of the Meiocene. We have evidence of numerous slight 
elevatory movements on the eastern coast of North America, and 
the various terraces of our rivers seem to present the same phe- 
nomena ; for the source of these elevatory movements we must 
look to the great volcanic foci of Greenland. 2 
Prof. Locke mentioned a locality in Ohio, at which the lime- 
Stone is ground down to a perfect plane, as if it had been done by 
4 stone-cutter by grinding one stone on another, over an extent 
often acres. Upon this planished surface, lines have been en- 
gtaved in systems perfectly straight and parallel, running from 
horthwest to southeast. Some of these lines are fine, as if cut 
With the point of a diamond, and others perhaps half an inch 
broad, and one eighth of an inch deep, scaled rough in the bot- 
tom, as if they had been ploughed by an iron chisel properly set 
and carried forward with an irresistible force. Prof. L. inferred 
from the facts of the exact straightness and parallelism of these 
lines, that they had been formed by a body of immense weight, 
Moving with a momentum scarcely affected by the resistance 
offered by the cutting of the grooves. Such a momentum and 
