190 Putnam — Results of Pendulum Observations. 



at Laredo and only *003 greater than at Austin, or an average 

 excess of '010.* As far as it is safe to draw an inference from 

 so small a number of observations (and considering the unfav- 

 orable location of the station at New Orleans), gravity is appar- 

 ently slightly in excess near the Gulf coast as compared with 

 interior stations. A determination nearer the present deltaf of 

 the Mississippi would be of interest in this connection, as well 

 as observations in the same latitude on the other coasts of the 

 continent. The smallness of the differences found indicate a 

 close approach to the condition of hydrostatic equilibrium in 

 this region. The fact that the slight excess is at the coast sta- 

 tions points possibly to some retardation in reaching this 

 condition. It is interesting to compare the conclusion 

 reached by Mr. McG-ee in discussing the condition of the same 

 region as deduced from geological and other considerations, in 

 these words : " So the data relating to the condition of the 

 earth's crust derived from the modern Gulf of Mexico indicate 

 that throughout the vast geologic province of southeastern 

 North America, isostasy is probably perfect, i. e., that land and 

 sea bottom are here in a state of hydrostatic equilibrium so 

 delicately adjusted that any transfer of load produces a quanti- 

 tatively equivalent deformation. "f 



While coast stations have in general shown less apparent irreg- 

 ularity in the force of gravity than is often found in the 

 interior, doubtless because they are situated so near the sea 

 level and are therefore more free from the uncertainties asso- 

 ciated with the sea level reduction, still greater discrepancies 

 appear than can be attributed to errors of observation. In the 

 following table are collected the residuals observed minus 

 computed gravity, for the stations on or near the coasts, thus 



* The attraction of an extended horizontal plate of rock of average density 31 

 feet (9-4 meters) thick corresponds to '001 dyne force of gravity. 



f The interesting fact has been recently brought out by an Engineer officer, 

 that a rise of one foot, since 1877, in the level of mean high water in the Gulf of 

 Mexico is indicated by the tidal observations at Port Eads at the delta of the 

 Mississippi. This change would correspond to a subsidence of the land of like 

 amount, but may possibly be accounted for by a local settling near the bench 

 mark. 



X " The Gulf of Mexico as a Measure of Isostasy," W. J. McGee, this Journal, 

 Sept., 1892, vol. xliv, p 189. The following conclusions are also quoted as bear- 

 ing on the same subject: 



" It appears that the crust, in the form in which it exists, must be in a condi- 

 tion of approximate hydrostatic equilibrium, such that a considerable addition of 

 load will cause any region to sink, or any considerable amount denuded off an 

 area will cause it to rise." "Physics of the Earth's Crust" (2d edition), by 

 Rev. O. Fisher, p. 355. 



" It may be laid down as a general rule that where great bodies of sediment 

 have been deposited over extensive areas, their deposition has been accompanied 

 by a subsidence of the whole mass." " On some of the greater Problems of 

 Physical Geology," by Major C. E. Dutton, Phil. Soc. of Washington, vol. xi, 

 p. 55. 



