McGee — Gulf of Mexico as a measure of Isostasy. 179' 



in or near zones of active vulcanism or orogeny, and (3) depo- 

 sition tracts in generally stable regions far removed from 

 Pleistocene glaciation, vulcanism and orogeny — it will be found 

 that the greater part of the third category are also tracts of 

 rapid subsidence or of rapid encroachment of the waters. The 

 data derived from this relation, which also are sometimes 

 equivocal though often unmistakable, are of two kinds, which 

 may be called respectively quantitative and qualitative / the 

 first being actual measures, and the second inferences from 

 analogous measured examples and from known geologic pro- 

 cesses. 



A good example of the first of these kinds of data is the 

 southeastern shore of North sea, which is burdened beneath 

 detritus dropped from the Rhine, Maas and Scheldt, the "Weser 

 and the Elbe, and which has been sinking since the beginning 

 of local history ; the island of Batavia, inhabited in the days 

 of Tacitus is drowned ; Zuydee Zee was formed by an inva- 

 sion of the waters about the end of the 13th century ; again 

 and again towns and villages have been inundated and swept 

 from the face of the earth ; broad slices of ill-fated Heligo- 

 land are annually devoured by the sea ; the Netherland polders 

 (or dike-protected lands) are maintained by artificial embank- 

 ments which require raising from generation to generation 

 until now cultivated fields lie 7 to 10 meters below tide level ; 

 and the artificial embankments could not withstand the force 

 of the waves were they not themselves protected by much 

 larger natural embankments called dunes (analogues of the 

 " keys " of the American coast). The measured rate of sink- 

 ing along the Netherland coast ranges from 0*09 meter to 

 0'75 meter per century ; since 1732 the mean rate, according 

 to Girard, has been about 26 meter per century. * The rate 

 of sea encroachment on the lowlands cannot accurately be 

 determined, by reason of the artificial intervention which per- 

 mits cultivation of lands lying far below tide level ; the rate 

 of encroachment on the higher lands is measured b}^ the 

 destruction of Heligoland, which was 190 or 200 kilometers in 

 circuit in the year 800, 72 kilometers in 1300, only 6*5 kilo- 

 meters in 1649, f and is decimated annually. Another exam- 

 ple is the tract centering about New York bay though 

 extending from Long Island sound to Chesapeake bay, into 

 which is poured nearly all the sediment gathered from a many 

 times larger semi-ellipse in eastern United States, and in which 

 also the sinking is known to be rapid. According to the con- 

 servative estimates of the late Dr. George H. Cook, the direct 

 subsidence reaches and probably exceeds two feet per century. 



* Recherches sur l'lnstabilite des Continents, 1886, p. 168. 

 f Ibid, p. 31. 



