Geology and Physical Geography, 349 



strata, and on the raised sea margins of New England. The 

 Professor took a general view of the effects of river action as 

 contrasted with the waters of the ocean in eroding the terra- 

 queous surface, and pointed out the various kinds of action 

 as illustrated by the rivers of America, as well as those of 

 other parts of the world. His general conclusions were, 

 that the hollowing out of river channels, as well as that of 

 valleys through which they flowed, was in a considerable de- 

 gree owing to the eroding effects of their waters, and that 

 the time required for their erosion was in the ratio of the 

 hardness of the rocks over which they flowed. He was of 

 opinion, too, that the various kinds of terraces, and the so- 

 called sea margins, might be accounted for by the gradual 

 operations of rivers and the ocean, without having recourse 

 to the supposition of our sudden catastrophes. The Pro- 

 fessor also entered into an explanation of the diluvium or 

 drift, and the various layers of firm mud, gravel, and larger 

 pebbles of which it was composed, — stating that, where the 

 drift was found separated into these three states, he was 

 inclined to believe that the separation was effected by an 

 aqueous agency posterior to its original deposition, and that 

 this upper or newer portion might be characterised by the 

 name of modified drift. 



Professor Sedgwick was compelled to some extent to differ 

 from the opinions of the learned American geologist, and 

 maintained that the general course of rivers was entirely 

 dependent on the nature of the surface over which they 

 flowed, and that from the natural laws of hydraulics their 

 waters sought the readiest and easiest outlets for the cur- 

 rents, instead of wearing down and scooping out those chan- 

 nels, and argued that one great and marked peculiarity of 

 the present surface of the earth, whereby it was elevated into 

 mountain and depressed into valleys, was due to the great 

 and sudden igneous convulsions to which this surface had 

 been subjected. In respect to elevated sea-beaches, too, he 

 stated that these owed their origin to upraising of the land, 

 not to recession of the sea. He did not deny that local and 

 limited effects were being produced by river action in the 

 present time ; yet these, in respect to the magnitude of the 



