Uplift and Subsidence. 3 



we must extend our data over large areas in order to 

 eliminate the local effects of the topography. 



The theory of isostasy. 



Some years ago the idea was advanced that a mountain 

 mass had under it a deficiency of material that was prac- 

 tically equal in amount to the mass of the mountain and 

 that under an ocean area there was an excess of matter 

 equal in amount to the deficiency of material in the ocean. 

 This balancing of the mountains and oceans by deficiency 

 and excess of material, respectively, in the outer portion 

 of the earth was termed isostasy by Major C. E. Dutton. 

 No comprehensive tests of this theory were made until the 

 early part of this century when Prof. John F. Hayf ord 

 made his splendid investigation in the figure of the earth 

 and isostasy. There were many papers written on the 

 subject of isostasy prior to Hayf ord 's work, among the 

 most important being those by Putnam and Gilbert.^ 



The fundamental principle of isostasy is that, at some 

 depth below sea level, the pressure on equal areas is the 

 same throughout the earth. Hayf ord adopted this prin- 

 ciple but it was necessary for him to make other assump- 

 tions in order to carry on his investigations. His 

 assumptions were, first, that isostasy is confined to a 

 certain definite zone, with a uniform limiting depth, which 

 he termed the depth of compensation; second, that the 

 compensation is complete, that is, that it exactly equals 

 in amount the excess or deficiency at the surface ; third, 

 that the compensation is uniformly distributed with 

 respect to depth; and, fourth, that the compensation is 

 directly under the topographic feature. Hayford did 

 not believe that these assumptions are strictly true but he 

 thought them to be as logical as any other simple assump- 

 tions which are necessary to be made in order that the 

 vast amount of computations may be undertaken. 



Let us get a clear idea of how the topography and com- 

 pensation are used in the determination of the figure of 

 th earth. We must correct the astronomic observations 

 in order to eliminate the local attraction of the topography 

 and the compensation, and after these corrections have 

 been applied we have data from which to determine the 

 lengths of degrees of longitude and latitude, at various 



'Appendix 1, U S. Coast and Geodetic Survey Eeport for 1894; and vol. 

 13, Bulletin, Washington Philosophical Society. 



