Association of American Geologists. 175 



Prof. Henri/ 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 — and so 

 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. 



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 

 a stone-cutter by grinding one stone on another, over an extent 

 of ten acres. Upon this planished surface, lines have been en- 

 graved in systems perfectly straight and parallel, running from 

 northwest 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 



