42 GEOLOGY OF THE NARRAGANSETT BASEST. 



The modern school of topographical geologists is disposed to explain 

 such differences as those which we are considering by the supposition that 

 the region of less relief — the eastern — has been long base-leveled, without 

 the refreshment of its relief which is induced by a subsequent process of 

 reelevation; while the western district, having been once, or perhaps more 

 than once, worn down to near the ultimate erosion plane, was lifted again 

 to a height which permitted the machinery of its torrents to sculpture new 

 reliefs. In favor of this supposition there is the fact that the summit levels 

 of many peaks in the West Appalachians are so nearly in one plane that it 

 is not unreasonable to suppose, as a 'working hypothesis, that the valleys 

 have worn down from an ancient base-level. To this suggestion it may be 

 answered that, so far as the evidence goes, there is reason to believe that 

 the eastern shore has shared in these upward movements. The Berkshire 

 Hills show, by the coincident levels of their summits, as distinct a trace 

 of base-level as do the Alleghenies. Moreover, in the immediate vicinity 

 of the Narragansett Basin the broad ridges of the Worcester axis carry its 

 levels to about 1,000 feet. Yet it is plain that this set of folds owes its 

 origin to the same movements that developed those of the Bay district, In 

 a word, even if we allow that uplift after base-leveling in the one case and 

 lack of the upward movement in the other might account for the very great 

 difference in conditions, we have not the means to verify the hypothesis; it 

 therefore has no apparent value to us in interpreting this field. 



Although I regard the considerations which are commonly included 

 under the title of "base-leveling" as one of the most important contributions 

 to physiographical geology, it seems to me that we must guard against the 

 danger of inferring too much concerning the existence of ancient leveling 

 of the land down to near the plane of the sea from the seeming accords in 

 the altitudes of mountain summits. It is easy to see that this accord is only 

 of a very geiieral nature, it being necessary in the classification to allow a 

 range of elevation amounting to several hundred feet, It may well be that, 

 beginning with the utmost diversity which could have existed in the heights 

 of the Alleghenies, the process of downwearing might have brought about 

 as near an approach to uniformity of height as actually exists in the peaks 

 of that range. So long as the rocks are of like hardness and the folds of 

 like size, the tendency would be to keep the downwearing crests at some- 

 where near the same level. 



