THE 



AMERICAN JOURNAL OF SCIENCE 



[THIRD SERIES. 1 



Art. XXV. — The Gulf of Mexico as a Measure of Isostasy / 

 by W. J. McGee. 



The now venerable James Hall was one of the first geolo- 

 gists to observe that areas of rapid deposition are areas of 

 subsidence. When Powell extended his early surveys into 

 western America he observed the 'converse relation, namely, 

 that areas of degradation are areas of elevation. As the geo- 

 detic surveys of Great Britain, India and the European conti- 

 nent yielded data for determining the distribution of density 

 in the earth, Pratt and afterward Fisher and others observed 

 that the sea bottoms are heavy, the continents lighter, the 

 mountain ranges lightest of all. Meantime and subsequently 

 the original observations were repeated and extended in dif- 

 ferent lands until the observed relations were found to be 

 general ; meantime also the relation of coexistence was inferred 

 to be one of sequence, and thus it came to be recognized that 

 mountains are high because they are light, that sea bottoms 

 are low because they are heavy, that areas of degradation rise 

 because of unloading, and that areas of deposition subside 

 because of loading — i. e. it came to be recognized that the 

 entire terrestrial crust is in a condition analogous to that of 

 hydrostatic equilibrium. This subject has been profoundly 

 studied by Dutton, who invented the term isostasy to denote 

 such condition of static balance in the external portion of the 

 earth. 



Am. Jour. Sci.— Third Series, Vol. XLIY, No. 261.— Sept., 1892. 

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