228 H. W. Pearson — Oscillations of 8ea-leveL 



now piled, certainly 5 feet (probably much more on the coast of 

 Norway) above the normal sea-level, and that the waters on the 

 eastern borders of the United States are correspondingly depressed. 

 It follows, therefore, that if the Gulf Stream — that force which 

 now restrains these waters in their abnormal position — should 

 decrease but slightly in its velocity of flow, the oceanic surface 

 would at once return in part towards that normal level from which 

 it has so long been displaced ; in other words, Scandinavia and the 

 British Islands would enter upon an epoch of upheaval, the Carolinas 

 upon an epoch of subsidence. As we have seen, the recent 

 protrusions of the north renders certain the fact that a great mass of 

 water has recently disappeared from this hemisphere. The transfer 

 of this water to the south makes an equal certainty that coexistent 

 with this removal all northward-flowing currents should have 

 decreased in their velocity of flow. It is clear, therefore, that these 

 opposing motions in our coastlines can be reduced to law and fore- 

 told in advance of observation. 



We have reason to believe, however, that apparent upheavals or 

 subsidences due to this cause will not at any time exceed 2 or 

 3 feet in vertical movement, and they consequently are of little 

 importance as compared with the periodic vibrations of 15 or 

 20 feet over an entire hemisphere, as developed herein. It never- 

 theless is important to detect and eliminate these minor deviations, 

 when we attempt the general investigation of coastal phenomena. 

 For a more extended, although still very incomplete discussion of 

 Ferrell's law, see the American Geologist, as before mentioned. 



To those who may wish to extend these investigations — and there 

 is great opportunity for such extension — caution should be given 

 against relying on evidence as to changes in the sea-level gathered 

 near the mouths of great rivers or in deltas like those of the 

 Nile, Po, Ehone, Ehine, Mississippi, etc. These delta deposits, 

 independent of their surroundings, are all sinking bodily and 

 spreading laterally, probably under some process of disgorgement 

 of their water contents. Much evidence of this exists. For instance, 

 E. L. Corthell says the delta lands of the Mississippi are unstable 

 both in vertical and lateral direction. A base-line 700 feet long, 

 measured accurately, had in five years increased to 712 feet. He 

 also quotes from the Eeport of the Mississippi Eiver Commission : 

 " Discrepancies in beach marks, level heights, and gauges could 

 only be satisfactorily accounted for by the most plausible explanation 

 of the subsidence of the whole delta" {The National Geographic 

 Magazine, December, 1897). 



M. Staring is of the opinion that the gradual depression of Holland 

 " is caused only by the subsidence of the alluvial ground, the weight 

 of the dikes, and the incessant passage of men and cattle " (Eeclus, 

 "The Earth," p. 547). Eegions like the northern and western 

 shores of the Adriatic, the deltas of the Ehine and Mississippi, should 

 thus be avoided ; the settlement of these delta deposits may obliterate 

 the vertical movements in the aqueous envelope ; observations made 

 along rock-bound coasts only are trustworthy. 



