McOee — Gulf of Mexico as a measure of Isostasy. 185 



limited portion of Adriatic Sea ; yet the value of this example is 

 diminished by reason of the vulcanism and orogeny of contigu- 

 ous territory. For similar reasons and because of its irregularity 

 of outline and inequality of loading in different parts, the 

 Mediterranean may be excluded from the list of noteworthy 

 deposition tracts of the globe. The African and Australian 

 degradation tracts may also be neglected, partly because the 

 areas are ill determined, partly because deposition is seldom 

 concentrated in measureable tracts. 



Weighing the various considerations affecting the value of 

 the data yielded by each of the tabulated deposition tracts, 

 they appear to fall into the following order : (1) Gulf of Mex- 

 ico, (2) North sea, (3) New York bay, (4) Bay of Bengal, (5) 

 Adriatic sea, (6) Hwang-hai, (7) Black and Azof seas, (8) Ara- 

 bian Gulf, (9) the Amazon estuary, (10) la Plata estuary, (11) 

 Caspian sea, (12) the Aral, and (13) Gulf of St. Lawrence. 

 On reviewing the data yielded by the several examples in 

 view of this weighing, in view of the ratios between areas of 

 degradation and areas of deposition, and in view of the relative 

 activity of the several rivers, the influence of tides and cur- 

 rents, etc., it appears that all are consistent — that every con- 

 siderable deposition tract beyond the reach of Pleistocene 

 glaciation, vulcanism, and orogeny is subsiding ; that, other 

 things equal and so far as the data are available and trust- 

 worthy, the rate of subsidence is proportional to the relative 

 areas of degradation and deposition ; and that, other things 

 equal and so far as the data are available and trustworthy, the 

 subsidence is proportional to the activity of the rivers in the 

 correlative degradation tracts. 



So the indirect data concerning isostasy, derived through 

 inferences from formations deposited or terranes degraded 

 during long past eons, are supplemented by a trustworthy 

 body of direct data derived from the physiography of the 

 earth in its present condition ; and the direct data are superior 

 to most* of the indirect in that they are susceptible of relative, 

 and in some cases absolute, evaluation. 



II. 



Even on casual inspection it is apparent that the Gulf of 

 Mexico is one of the most fortunately situated deposition tracts 

 of the globe for the measurement of isostatic subsidence ; for 

 it is a land-rimmed basin of considerable area, connected with 

 open sea through relatively narrow straits, and fed by drainage 



* In one case the indirect data have been evaluated. This is Gilbert's classic 

 study of the strength of the earth's crust as indicated by the deformed shore lines 

 of the extinct Lake Bonneville (Bull. Geol. Soc. Am., vol. i, 1889. pp. 23-27; 

 Monograph I, U. S. Geo). Survey, ''Lake Bonneville," 1890, p. 387, et seq.). 



