502 PROCEEDINGS 017 COLUMBUS MEETING. 



cerning isostasy derived from the geologic record arc supplemented by a trust- 

 worthy body of direct data derived from the physiography of the earth in its 

 present condition ; and the direct data arc superior to most of the indirect in that 

 they arc susceptible of relative, and in some instances positive, evaluation. 



The Gulf of Mexico is one of the most fortunately situated deposition tracts of 

 the globe for the measurement of isostatic subsidence in that it is a nearly closed 

 land-rimmed basin of considerable area fed by drainage from a many times larger 

 degradation tract j and, moreover, the sedimentation is not confined to a single 

 delta, but is distributed in simple and easily ascertained fashion. Now, the Gulf 

 coast has only recently been surveyed with precision, and the surveys have not 

 been repeated in such way as to give quantitative rate measurements of movement ; 

 but the physiographic evidence of subsidence is unmistakable and indicates a mean 

 rate not less, and probably more, than a foot per century. This rate is somewhat 

 less than the estimated degradational transfer of material requires. Moreover, the 

 physiographic indications of subsidence vary in strength about different parts of 

 the coast; they are weakest in the northeast, where the affluents are short and 

 feeble ; stronger in the northwest, where the affluents are longer and more potent ; 

 strongest in the north about the delta of the chief river of the continent. In brief, 

 if the Gulf of Mexico be considered as a unit, its shores appear to he subsiding 

 about as rapidly as isostasy demands; and considered as an assemblage of deposi- 

 tion tracts, the local rates of subsidence appear to be delicately adjusted to the local 

 rates of deposition. Accordingly, the data yielded by this fortunately situated 

 deposition tract indicate that throughout the vast ideologic province of southeast- 

 ern North America isostasy is probably perfect, i. <\, that land and sea bottom are 

 here in a state of hydrostatic equilibrium so delicately adjusted that any transfer 

 of load produces a precisely equivalent deformation. 



It is well known that the later formations of the Gulf province (notably the 

 Columbia and Lafayette formations) represent continental oscillations reaching 

 several hundred feet in amplitude. Now in contrasting these great oscillations 

 with the gentle modern movement of the coast, they are found to differ widely ; 

 the modern subsidence is a gentle warping in such direction as to deepen the basin 

 and gradually submerge its perimeter, while the old oscillations were wide-spread 

 and involved both sea-bottom and continent; the modern movement is slight and 

 commensurate with the simple and uniform processes of erosion and sedimentation, 

 while the old movements were cataclysmic and utterly transcended the influence of 

 rain and rivers. Accordingly, while the modern movements give abetter measure 

 than has been obtained elsewhere of the efficiency of degradational transfer of 

 matter as a cause of deformation, the movements recorded in the Columbia and 

 Lafayette formations were of so much greater amplitude that they may not be 

 referred to a similar cause; therefore in this province, as in others, it becomes 

 necessary to discriminate the two classes of earth movements elsewhere called 

 respectively antecedent and consequent. So the modern province measures the com- 

 petence of isostasy, the ancient province its incompetence ; the modern Gulf illus- 

 trates the magnitude, the ancient Gulf the minitude of isostatic deformation as a 

 means of continent-making. 



Although isostatic action alone is incompetent to explain the great continental 

 oscillations attending the deposition and degradation of the Columbia and Lafayette 

 formations, certain peculiarities in these oscillations maybe hypothetic-ally explained 

 through the doctrine of isostasy. During the low-level periods represented by the 



