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



III. 



The measure of isostasy found in the modern Gulf of 

 Mexico might be tested b} T comparison with the measures 

 yielded by other gulfs and bays, were the data concerning the 

 several fortunately situated deposition-tracts sufficiently pre- 

 cise ; but in the dearth of surveys and bench-marks it must 

 suffice to repeat that the physiographic data afforded by the 

 Bay of Bengal, the Adriatic, Hwang-hai, Black and Azof seas, 

 the Arabian gulf, the mouths of the Amazon and la Plata, 

 etc., are harmonious therewith. The measure cannot well be 

 tested by comparison with the indirect data derived from study 

 of ancient formations or terranes in other geologic provinces, 

 since most of such data are isolated and, by reason of ignorance 

 concerning the absolute or relative areas of the tracts of degra- 

 dation and deposition respectively only rudely qualitative, and 

 since in the single case in which the indirect measure is quanti- 

 tative — Gilbert's ancient Lake Bonneville — the problem is so 

 complex as to permit only the statement that the results are in 

 a general way harmonious with the data yielded by the modern 

 Gulf. The measure may, however, be checked by comparison 

 with past records of the geologic province represented by the 

 Gulf and adjacent lands. 



The later geologic history of the Gulf of Mexico is now 

 fairly well known. The uppermost structural unit of the con- 

 tiguous land area is the early Pleistocene Columbia formation ; 

 the next in age is the late Neocene (probably Pliocene) Lafay- 

 ette (Appomattox) formation. Now the history recorded in 

 these formations and in the unconformities by which they are 

 bounded may be thus interpreted : Before the Lafayette period 

 the configuration of the southeastern quarter of the continent 

 was much the same as to-day, save that the land lay somewhat 

 lower and flatter ; then came the Lafayette inundation, 300 to 

 900 feet in depth and extending from 100 to 500 miles inland; 

 this was followed by a high leA^el period during which the land 

 was tilted seaward from the Appalachian axis but lifted 300 to 

 1000 feet higher than before (or now), and during which half 

 of the volume of the Lafayette formation was degraded, while 

 the rivers carved broad and deep canyons forming the estuaries 

 yet indenting the Atlantic and Gulf coasts ; next the land 

 gradually subsided to its previous (and about the present) mean 



tion of terms to which are due the apparent discordance between the current 

 doctrine of physics and that of geology, it may be pointed out that these re- 

 markable and admirable inductions of physics have no more to do with the 

 deformation dealt with by geologists than laboratory experiments on the rigidity 

 and brittleness of ice have to do with the movements of glaciers ; and the 

 mobility of the terrestrial crust through a range reaching thousands of feet and 

 even miles is now quite as well established as is the mobility of glacier ice. 



