184: McGee — Gulf of Mexico as a measure of Isostasy. 



ceeding rapidly, but the correlative area of loading is so ill-de- 

 fined as hardly to be susceptible of estimate ; while in the fourth 

 area (that of la Plata), where degradation is probably more slug- 

 gish, the deposition area is still more indefinite. The fifth in ex- 

 tent of the degradation tracts concentrates its detritus in the well 

 defined area of Black and Azof seas ; yet so little is known of the 

 coastal configuration of these seas and of the .possible influence 

 of Pleistocene glaciation and contiguous orogeny that the data 

 afforded by this example are of little service. The deposits of 

 the sixth degradation tract are in like manner concentrated 

 upon a limited area which may be assumed to equal the north- 

 ern half of Caspian sea, and in the seventh tract the concen- 

 tration is still greater, the degradation tract tributary to the 

 Aral being no less than twenty-four times that of deposition ; 

 but both examples are enclosed basins in which the record of 

 isostatic subsidence is complicated by the direct displacement 

 of the water and also by the variations in water-volume depend- 

 ing on climatal conditions, and accordingly the data in these 

 cases are unworthy of trust. The eighth degradation tract 

 (Ganges and Brahmaputra) is especially noteworthy by reason 

 of the activity of the rivers and the vast volume of detritus 

 annually discharged ; but the correlative deposition area is so 

 ill defined that, apart from the incomplete measurements of the 

 Indian survey, the relative value of the data afforded by this 

 example is not easily ascertained ; and this is true in still 

 stronger measure of the ninth tract, or that of the notably 

 active Indus. The tenth degradation tract in extent (St. 

 Lawrence) is useless as a measure of isostatic subsidence, (1) 

 because it is wholly within the area of Pleistocene glacia- 

 tion, (2) because the correlative deposition tract is ill-defined, 

 and (3) because its course is interrupted by several settling- 

 basins. The eleventh tract (that of the Rhine and its neigh- 

 bors) is one of activity in both degradation and deposition, and 

 one moreover in which the combined effect of tides and cur- 

 rents probably tends to confine deposition to the comparatively 

 narrow zone along which subsidence is best marked, if not 

 chiefly to Zuyder Zee and the riparian estuaries ; yet it is possible 

 if not probable that this area is within the influence of Pleis- 

 tocene glaciation, and hence of an extraneous disturbance of 

 equilibrium not yet completely restored. The twelfth tract, 

 too (New York and neighboring bays), lies partly within the area 

 of glaciation, and is moreover affected by a modern displacement; 

 but it acquires value from the connection with rude yet useful 

 rate measurements. The smallest of the degradation tracts (that 

 of the Po, the Adige , the Piave, and several smaller streams, 

 all of great activity), concentrates its products in a presumptively 



