McGee— Gulf of Mexico as a measure of Isostasy. 189 



sissippi sound ; west of the great delta the corresponding keys 

 are completely submerged and reduced to a series of banks and 

 shoals separated from the mainland by a trough even broader 

 than Mississippi sound,, and new keys and sounds are forming 

 along the present coast ; southwest of Galveston bay the keys 

 are of unparalleled strength and continuity, while the sounds are 

 broad and scores or even hundreds of miles in length. More- 

 over, some rivers are estuarine while others are delta-builders, 

 and this difference in habit is evidently independent in large 

 measure of stream- volume and of sediment. Yet on careful ex- 

 amination, the apparent discordance falls into harmony : The 

 strength of keys and width of sounds is least in the eastern 

 part of the embayment where the sediment-bearing rivers are 

 relatively short and feeble, greater in the northwest where the 

 rivers are longer and more potent, and greatest about the de- 

 positing ground of the chief river of the continent. So, too, 

 all the large rivers of the western and eastern coasts are delta- 

 builders ; while all of the extra-Mississippi streams within a 

 hundred or a hundred and fifty miles of the great river (and 

 presumptively within reach of its isostatic influence) embouche 

 into estuaries.* This harmony is suggestive, if not precisely 

 indicative, of a quantitative relation between local deposition 

 and local subsidence. 



Summarily, the Gulf of Mexico, considered as a unit, is one 

 of the most fortunately situated deposition-tracts of the world 

 for yielding a measure of isostatic subsidence; considered 

 again as a unit, its shores appear to be subsiding quite as 

 rapidly as isostasy demands ; and considered as an assemblage 

 of deposition subtracts, the varying rates of subsidence appear 

 to be delicately adjusted to the local rates of deposition. 



So the data relating to the condition of the earth's crust 

 derived from the modern Gulf of Mexico indicate that through- 

 out the vast geologic province of southeastern North America, 

 isostasy is probably perfect, i. e., that land and sea bottom are 

 here in a state of hydrostatic equilibrium so delicately adjusted 

 that any transfer of load produces a quantitatively equivalent 

 deformation, f 



* These features are set forth at some length in a paper on the Lafayette for- 

 mation, 12th Ann. Rep. U. S. Geo!. Survey. 1892, pp. 347-521, pis. xxxii-xli. 



f Thomson and other physicists concluded some years ago, after a study of the 

 tides, precession, etc., that the earth as a whole must be as rigid at least as steel. 

 Newcomb has quite recently concluded, after discussing new data (including 

 Chandler's brilliant coordination of the recorded variations in latitude in connec- 

 tion with Oppolzer's computations), that " the earth yields slightly less . . . than 

 it would if it had the rigidity of steel, and that it is consequently slightly more 

 rigid than steel" (Monthly Notices of the Royal Astronomical Society, vol. lii, 

 no. 5 (1892), p. 339); so that while Thomson's result gives a minimum value, 

 that of Newcomb gives a maximum value for the rigidity of the earth as deter- 

 mined from cosmic relations. Now, without analysis of the differences in defini- 



