EFFECTS OF WAVE ACTION. 47 



that of the last ice epoch, on this field was limited. We may therefore 

 assume, what is inferred from other evidence, that the drainage of the 

 district is substantially the same as it was before the last advent of the 

 glaciers. The drainage consists of sundry deep channels, the arms of 

 Narragansett Bay and their continuations in the narrowed rivers. To 

 explain these stream beds, we must assume that the surface of the country 

 was considerably higher during the preglacial time than it is at the present 

 dav. If, as is probably the case, the central part of the bottom of Xar- 

 rao-ansett Bay is filled in with mud to the depth of 100 feet or more, as is 

 the case with other channels of like character on this part of the coast, 

 then the recent subsidence may exceed 300 feet. A like process of reason- 

 ing applied to other parts of the shore between the Delaware and the St. 

 Lawrence leads to approximately the same conclusion as to the amount in 

 which the sea has gained on the land. It should be said, however, that this 

 change in the position of the shore may be due to an alteration in the level 

 of the sea itself quite as well a? to the lowering of the land in this part ot 

 the shore; in fact, the extent of this modern invasion of the land by the 

 sea along nearly all the shores of the continents raises the presumption that 

 the acti< >n may have been due to a vast movement of the floor in some part 

 of the ocean realm. 



GENERAL STATEMENT CONCERNING BASE-LETBLIXG. 



What has been said in the preceding pages concerning the relations of 

 marine and laud denudation makes it desirable to assemble the considerations 

 which bear upon this problem. 



There can be no question as to the importance of the base-leveling 

 theory, which assigns to the atmospheric agents of erosion the downwearing 

 of the land masses. It should be noted, however, that the marine agents — 

 the cutting action of the waves and the marine currents dependent on wind 

 and tidal work — have in their appropriate place a certain amount of influ- 

 ence. It should also be noted that as the land is worn down toward the 

 level of the sea the efficiency of the atmospheric forces in the work of 

 further reduction continually diminishes, because of the lessened fall of the 

 streams and from the tendency of the surface to become deeply covered 

 with a protecting detrital envelope. 



In the low levels of the land, where the aerial agents become less 



