48 GEOLOGY OP THE NARRAGANSETT BASIN. 



effective, we may always expect to find some effect arising from the repeated 

 visitations of the sea brought about by the almost continual oscillations in 

 the height of the land. The measure of this marine work is commonly the 

 greater the nearer we attain to the average level about which the sea has 

 oscillated for a considerable time. Thus marine action comes in to sup- 

 plement that of the atmosphere. 



Along the coast of New England, and particularly in the district con- 

 sidered in this report, within the limits of the recent oscillations of marine 

 level, we find at many points evidence that the sea at higher stations than 

 now worked to remove the coating of detritus and to expose large areas 

 of the surface to the process of decay, which rapidly tends to break up the 

 rocks. This part of the marine work, in favoring erosion, is perhaps of as 

 much consequence as that due to the direct cutting action of the sea. 



It may be remarked that the frequent invasions of the sea, by producing 

 plains of detrital material, such as those which exist in the southern part of 

 the United States, tend also to reduce the surface of the land to an 

 approximately level form. Thus the evidence goes to show that beneath 

 the southern plain the contour of the ancient rocks is irregular, they having 

 been mantled over by a thick coating of ddbris accumulated along the 

 continental shelf. We therefore see that the ocean tends in two diverse 

 wavs to bring about coastal plains — first, by aiding in wearing down origi- 

 nally irregular surfaces to a level attitude ; and secondly, by constructing 

 detrital plains over those surfaces which in part have thus been brought to 

 a nearly horizontal attitude. 



As for the oscillations of the land which serve to bring the mill of the 

 surf at various levels over its surface, it may be said that since the Carbon- 

 iferous period there is evidence of many such swingings, which have brought 

 the plane of the sea from a few score to several hundred feet above its 

 present position on the Atlantic base of the continent. The evidence to 

 the same effect from other regions is so extensive that it may be called a 

 world-wide phenomenon. It may safely be assumed that coast lines are 

 normally instable, and this through a range of several hundred feet. 



As to the amount of cutting which can be effected by the sea in pro- 

 portion to that which may be accomplished by the descent of waters from 

 a high level to the shore line, the facts are not yet sufficiently ascertained 

 to permit any definite statement. It may be said, however, that where the 

 exposure is such that the waves may assault the shore with considerable 



