W J Mcgee — A measure of isostasy. 501 



warded to me this day, and I desire you to convey to the Society my grateful 

 appreciation of its sympathy and good wishes. 



I regret more than ! can tell you my inability to add anything whatever to the 

 pleasure or profit of the Columbus meeting, to which I have been Looking forward 

 with high expectations for the last six months. 



I rejoice with you in every addition that is being made to our knowledge of 

 American geology. I count it a great honor and privilege to have been able to con- 

 tribute even in the humblest degree to its advancement, but no one realiz.es more 

 distinctly than I do at this time how small a part the contributions of even the 

 most gifted member of our profession make to the wide and ever widening river of 

 our knowledge. I am sure that we all recognize the fact that our work so far is 

 mainly limited to the headsprings of the river. 



1 close with the sentiment, "The Geological Society of America, esto perpetua." 

 Very truly yours, Edward Orton. 



The first paper read was entitled: 



Till-: GULF OF MEXICO AS A MEASURE OF ISOSTASY. 

 BY W .r MCGEE. 



[Abstract.'] 



The term " isostasy" was coined by Dutton to denote a condition of static equi- 

 librium in the terrestrial crust, in virtue of which areas of degradation rise and 

 areas of deposition sink. The earlier data on which the doctrine of isostasy 

 depends were indirect, i. e., they were inferences from ancient formations and old 

 surfaces; but it is now found that the modern continental movements affecting 

 areas of deposition yield direct data sustaining the doctrine. Such data maybe 

 either quantitative, when the rate of movement is measured, or qualitative, when 

 movement is ascertained but not measured. The most trustworthy measured ex- 

 amples are (1) the Netherland coast, which has been under observation for a mil- 

 ieu i in n and which is subsiding beneath the sediments of the Rhine and its neighbors 

 at a rate varying from 0.09 to 0.75 meter per century, the mean since L732 being 0.26 

 meter; and (2) the New Jersey coast, which is subsiding beneath the sediments of 

 the Hudson and Delaware at the rate of about two feet per century. Scarcely less 

 decisive evidence of subsidence, though at unmeasured rates, is yielded by every 

 noteworthy deposition tract of the globe (exclusive of Africa, where the data are 

 inadequate), including the embouchures of the Amazon, the Yang-tse-kiang, 

 1 1 wang-ho, la Plata, the European rivers embouching into the black and Azof seas, 

 t he Volga and I Iral, the Syr I (aria and Amu Daria (together feeding the Aral sea . 

 the Ganges and Bramaputra, the "Five Rivers" headed by the Indus, the Saint 

 Lawrence, the Po and its neighbors, and the Mississippi. On reviewing this evi- 

 dence ii appears that every considerable deposition tract beyond the reach of Pleis- 

 tocene glaciation, vulcanism and orogeny is subsiding; that, other things equal and 

 so far as the data are available anil reliable, the rate of subsidence is proportional to 

 the relative areas of degradation and deposition ; and thai, ot her things equal and 

 ho far as the data are available and reliable, the subsidence is proportional to the 

 activity of the rivers in the correlative degradation tracts. So the direct data con- 



I..W III Bi'i.i., 9oi . Vm., Vol. 3 1891. 



