180 JfoGee — Gulf of Mexico as a measure of Isostasy. 



The encroachment of the bays and of the adjacent Atlantic 

 is proportionately rapid ; within the last fifty years it has 

 reached half a mile in some localities, averaging a furlong or 

 more, and has destroyed property amounting to millions of 

 dollars. Both of these deposition tracts lie near and possibly 

 within the influence of Pleistocene glaciation ; yet they are 

 especially significant in that the quantitative data derived from 

 them are connected with and give value to data of the qualita- 

 tive kind. 



Through the association of the two kinds of data on the 

 Netherland and Xew Jersey coasts, as well as through infer- 

 ence from geologic process, it is found that the most trust- 

 worthy physiographic indications of subsidence are : (1) en- 

 croachment of the sea ; (2) wave-built breakwaters along 

 lowland coasts (the "dunes" of Holland, the "keys" of 

 America) ; (3) precipitous and talus-free or undercut cliffs 

 along highland coasts; (4) estuaries at the mouths of sediment- 

 bearing rivers ; and (5) naturaj levees along the lower courses 

 of rivers, leaving low-lying and periodically flooded marshes or 

 salines on either hand. These physiographic indications 

 express rates, though only in a qualitative way: Thus, the rate 

 of encroachment is a function of the subsidence, but is affected 

 by the seaward inclination, by the obduracy of the terrane, by 

 the force of the waves and currents, etc. The building of 

 natural breakwaters is characteristic only of coasts skirted by 

 submerged terraces or shelves ; for when coasts are so condi- 

 tioned the waves come in with ever increasing impetus over the 

 subsiding sea bottom, constantly casting up sand to strengthen 

 the barrier, and if the contiguous land is low a lagoon or 

 "sound" is formed behind the barrier and widens more rap- 

 idly than the barrier, is driven inland until both are finally 

 overflowed and converted into open sea, when the process is 

 repeated on new-made shores; and thus in a general way the 

 strength of wave-built breakwaters and the width of the 

 lagoons separating them from the mainland give rude measures 

 of subsidence. Talus-free cliffs are indeed an indication of the 

 ever increasing force of the waves on subsiding coasts ; yet, 

 while the cliff configuration of subsiding tracts like Chesapeake 

 bay and of rising tracts like Puget sound are markedly dis- 

 tinct, so many other conditions affect sea-washed rock faces 

 that they are useful only in exceptional cases. Again, estu- 

 aries may be inherited from earlier eons, and at the best only 

 indicate that subsidence outruns sedimentation, thus giving 

 minimum rather than mean measures of movement. Levee- 

 building, on the other hand, may represent merely diminution 

 in declivity resulting from the pushing out of deltas, and on 

 the whole tends to give excessive or at most maximum meas- 

 ures of subsidence. 



