76 GEOLOGY OF THE NAKRAGANSETT BASIN. 



degradation of Iron Hill, as above noted, can be explained in this way. 

 It therefore seems reasonable to adduce ice work as one of the agents which 

 have served to bring about the destruction of the original topographic reliefs 

 of this district. 



Along* with base-leveling* and ice work, there is another class of agents 

 which have doubtless operated with much effect in bringing the district into 

 its planed-down state. These are the foi*ces which act at and below the 

 level of the sea. There can be no question that the effect of the surf and 

 the shore currents is to plane off the rocks and to bring* about such topo- 

 graphic conditions as are found in this basin. The only doubt is as to the 

 rate at which the work may go on. Judging by the speed with which the 

 benching action of the sea proceeds where the attack is delivered on hard 

 (i. e., undecayed) rocks, geologists have generally assumed that the aggre- 

 gate work which is due to this action is relatively small, that it plays no 

 important part as compared with base-leveling due to atmospheric agents. 

 We must remember, however, that what we know of the extent of super- 

 ficial decay in this and other countries requires us to believe that in the 

 oscillations of the continents it must often happen that deep sections of 

 rocks which have been made very friable are exposed to the mill of the surf. 

 In this case it is fair to presume that they might be swept away with some- 

 thing like the speed which is exhibited in the disintegration of the Pliocene 

 cliffs of Marthas Vineyard. When they face the open sea, these deposits, 

 in coherence comparable to the decayed beds of the Southern Appalachians, 

 are retreating at the rate of about 3 feet per annum, as determined by 

 fifty years' observations. At this rate the surf mill would be able to 

 work inward across the field of the Narragansett Basin in less than one 

 hundred thousand years. 



