422 



University of California. 



[Vol. i. 



greater deformations of the crust which many geologists would 

 allow to be isostatic in character, while vigorously protesting against 

 the idea of delicate adjustment to sedimentary loading, to which 

 McGee would restrict the term isostasy. 



The Bearing of Recent Pendulum Observations. — The recent 

 'gravity determinations recorded by Putnam and discussed by Gilbert 

 appear to tend toward this larger view of isostasy, indicating as 

 they do that the earth's crust rigidily upholds masses far greater 

 than the advocates of isostasy had formerly been willing to admit. 

 In his last paper, however, Mr. Putnam * finds gravity slightly in 

 excess near the Gulf coast as compared with interior stations. This 

 is interpreted by him as indicating a slight retardation in reaching a 

 state of perfect isostasy. The data are too meager to draw any very 

 definite conclusions as to whether the sediments of the Mississippi 

 are the cause of subsidence. Such observations would need to be 

 repeated in far greater number, not only for the Mississippi delta, 

 but for other areas of active deposition and subsidence before we 

 could be entitled to draw final conclusions from them. In the pres- 

 ent instance the stations are so few in number, and the differences in 

 gravity between the coast and interior stations so slight, that they 

 do not seem to give any definite testimony in favor either of inde- 

 pendent or isostatic" subsidence. Moreover, confidence in the 

 result, where such small differences are at stake, is impaired by the 

 t uncertainties involved in the theoretical reductions of the observa- 

 tions to a standard. For example, the difference between two sets 

 of observations at Austin, so reduced, is .002 dyne, while the differ- 

 ence between the observations at the coast station at Galveston and 

 one set of the inland observations at Austin is only .001 dyne. 

 Moreover, all the observations show a theoretical defect of gravity, 

 averaging,according to Bouguer's reduction, .05 18 dynes, and accord- 

 ing to Faye's, .038 dynes. This would rather indicate that the whole 

 region has subsided more than isostasy demands. 



Highly interesting in this connection are the pendulum observa- 

 tions which have been carried on in Europe during the last few 

 years by von Sterneck f and others. These observations reduced, 



"Am. Jour. Science, Vol. CLI (1896), pp. 386-192. 



t Rev. in Neues Jahrbuch f. Min. u. Pal., 1896, Bd. I, s. 234-239. 



v 



