Me Gee — Gulf of Mexico as a measure of Isostasy. 187 



physiographic indications of land-subsidence are numerous, 

 consistent, and unmistakeable. Thus, island after island along 

 the Louisiana coast has been submerged bodily or devoured by 

 slices, and many historic plantations on the shores of Missis- 

 sippi sound and the open Gulf have melted into the waters 

 within the present century ; thus, too, the coasts (with one or 

 two most significant exceptions) are skirted by wave-built 

 breakwaters separated from the mainland by long narrow bays 

 (the "keys" and "sounds" of the vernacular) from the Rio 

 Grande to Florida strait ; again, wherever the generally low 

 coastward lands rise a dozen feet or yards above tide, they are 

 carved into precipitous talus-free cliffs ; half of the rivers, too, 

 albeit heavily detritus-charged, embouch into estuaries ; and, 

 moreover, each principal river, whether estuarine or delta- 

 building toward its mouth, divides into distributaries (or 

 " bayous") in its lower reaches, and the anastomosing channels 

 are flanked by natural levees separating periodically-inundated 

 flats, which are lakes, marshes, salines, or " black prairies " ac- 

 cording to local conditions. In addition to these physiographic 

 data there are found buried forests, old causeways now over- 

 flowed at low tide, aboriginal shell heaps nearly or quite sub- 

 merged, savanna pine forests once luxuriant but now poisoned 

 by salt water, and various other indications of changing rela- 

 tion between land and sea ; and it is to be observed that while 

 these indications vary in strength they are all consistent in 

 direction — all point to subsiding land, none point to rising or 

 even stationary land. 



On comparing the physiographic data about the shores of 

 the Gulf with the like data yielded by measured examples of 

 Hew Jersey and the Netherlands, it would appear, (1) that the 

 average rate of sea-encroachment about the Gulf is nearly or 

 quite as high as on the New Jersey coast and higher than on 

 the Netherland coast, at least since the building of the dikes ; 

 (2) that on the average the wave-built breakwaters are higher 

 and more distant from the mainland than in New Jersey, and 

 more extensive than, though scarcely so high and distant as in 

 the Netherlands ; (3) that the talus-free cliffs are even more 

 characteristic than in New Jersey or (probably) the Nether- 

 lands ; (4) that on the average the estuaries are nearly as broad 

 and deep as in New Jersey and more extensive than in Hol- 

 land ; and (5) that the natural levees are relatively higher and 

 broader than in New Jersey and, so far as comparison is possi- 

 ble, about as high and broad as in the Netherlands. The value 

 of the physiographic indications of subsidence of course de- 

 pends largely on a number of local and general conditions, such 

 as volumes of rivers, height of tides, strength of oceanic cur- 

 rents, direction of prevailing winds, the material, height, and 



