18S McGee — Gulf of Mexico as a measure of Isostasy. 



general configuration of shores, the breadth and depth of the 

 water-body in which the waves are generated, etc. ; yet howso- 

 ever these conditions affecting the comparison are weighed, the 

 Gulf phenomena appear to record an average subsidence fully 

 as rapid as that of the Netherlands or New Jersey ; or (the 

 vertical movement in the carefully measured case amounting 

 to 0"26 meter and in the more roughly measured example 

 reaching two feet per century) of at least a foot in a hundred 

 years. 



On comparing this value with the rates of degradation and 

 correlative deposition, a relation is obtained which, although 

 lacking in precision, is nevertheless useful : The area of deg- 

 radation is 1,800,000 square miles ; the maximum assignable 

 area of deposition is less than 300,000 square miles, and since 

 the direction and force of currents and the configuration of the 

 Gulf bottom alike indicate that appreciable sedimentation 

 must be confined to a relatively narrow zone skirting the coast, 

 it may be placed at a third of that area or 100,000 square 

 miles, which is one-eighteenth that of degradation. Now, the 

 commonly accepted rate of degradation of the earth's surface, 

 based on the surveys of the Mississippi by Humphreys and 

 Abbot, is a foot in 6000 years ; and this corresponds to deposi- 

 tion within the Gulf reaching one foot in 333 years. So it 

 would appear that the average rate of subsidence deduced from 

 comparisons with New Jersey and the Netherlands is three 

 times higher than would be required for isostatic adjustment ; 

 and although the various factors are so uncertain (probably 

 Humphrey's and Abbot's value and the deduced rate of sub- 

 sidence are too low and the assumed deposition area too high) 

 that the estimate is subject to a " probable error" perhaps 

 large enough to explain the discrepancy, the discrepancy never- 

 theless suggests that a part of the subsidence may be due to 

 some other cause. This suggestion derives strength from the 

 indications of general subsidence along the contiguous Atlantic 

 coast of the United States. 



On comparing the local physiographic indications of subsi- 

 dence at various points on the Gulf coast, a noteworthy diver- 

 sity is found. Thus, the most decisive historical records of en- 

 croachment come from the Louisiana and Mississippi coasts, and 

 there, too, talus-free cliffs are most characteristic. Again the 

 strength of keys and the width of sounds varies widely ; along 

 the Florida coast, despite the long sweep of storm winds and 

 trades, the keys are weak, the sounds narrow and shoal ; toward 

 the Appalachicola and Mobile bay the keys strengthen and 'the 

 sounds widen, save where dammed by deltas ; west of Mobile 

 bay the ke} 7 s are huge but half submerged banks rising in a 

 line of islands separated from the mainland by the broad Mis- 



