ABSTJ^ACTS III 



in his work on the pre-Cambrian crystallines of the northwest, in which 

 unconformities are given great prominence; by McGee in his investi- 

 gations of the coastal plain deposits of the Middle Atlantic slope, in 

 which similarity of genesis, or homogeny, is the governing factor; and 

 by Davis and others in physiographic work, in which periods of base- 

 leveling are made the all-important features in the cycle of land 

 degradation, and inferentially the consequent sedimentation in adjoin- 

 ing, seas. 



Without considering all the available physical and biotic criteria of 

 correlation in these various local and general phases it may be said 

 that general correlation finds a fertile suggestion for a rational foun- 

 dation in the factors which govern sedimentation. Its chief criterion 

 is a function of continental growth and decline, and is dependent upon 

 orogenic movement. This has recently be encalled correlation by 

 mountain-building cycles, or orotaxis. The division planes cutting 

 the geological columns into systems, series, or smaller parts, are actu- 

 ally, as well as theoretically, the lines of unconformities and their rep- 

 resentatives. In the case of the more extensive ones they do, no 

 doubt, represent base-leveled surfaces, or peneplains. 



Modified Drift in St. Paul, Minnesota. By Warren Upham. 



Within the limits of St. Paul (fifty-five square miles) are numerous 

 plateaus of drift gravel and sand, 225 to 240 feet above the Missis- 

 sippi River, and about 100 feet above the highest plains and terraces 

 of similar modified drift in the Mississippi valley. The courses of 

 marginal moraines in this city and its vicinity, the glacial strise, and 

 diverse origin of the glacial and modified drift on the east and west, 

 show that the slopes and currents of the ice-sheet converged to this 

 area from the northeast and the northwest. On account of the sig- 

 moid course of the Mississippi valley here, during the final melting 

 and retreat of the ice-fields the tract where they had been confluent 

 was occupied by a small glacial lake named Lake Hamline, which 

 extended over the greater part of the western half of the area of St. 

 Paul, having an outlet toward the southwest and south, across the 

 present watershed between the Minnesota and Mississippi rivers, to 

 Rich Valley and the Mississippi. In this temporary lake the modified 

 drift plateaus were accumulated, their material being thought to be 

 supplied from exceptionally abundant englacial drift brought to the 

 area of glacial confluence. The surface of the glacial lake during the 



