GEOLOGY. 241 



Since diastatic movements are predominantly vertical and measured 

 from sea-level as a datum plane, the determination of tlie mean sea-level 

 (commonly called the " ligure of the earth," or, more properly, the yeoid) 

 is imj)ortant to geologists. The " figure of the earth " is also important 

 to geologists in another way: It has long been known that despite the 

 gravitational attraction of mountains and continental masses a plumb- 

 line suspended at the sea-shore is generallj^ deflected seaward, and that 

 in some cases a plumb-line suspended at the base of a mountain range 

 is deflected toward the plain rather than the mountains ; and accordingly 

 it has been supposed by many physicists, astronomers, and geologists 

 that the ro(;ks constituting the ocean bottom are heavier than those of 

 the land, and that the rocks underlying plains are often heavier than 

 those forming mountain masses. Suess and some others have indeed 

 maintained that such mountain systems as the Andes must attract and 

 materially elevate the surface of contiguous ocean waters; but Pratt 

 and several other careful students have, on the contrary, advocated the 

 simi)le inference from observation; and Faye has attributed the great 

 iuecpialities of the earth's surface to the more rapid refrigeration and 

 consequent condensation of sea bottoms than of laud surfaces. Now, 

 the determination of the differences in density indicated by the anom- 

 alous deflection of the plumb-line in certain cases, and so the solu- 

 tion of some of the most profound problems with which geologists have 

 ever grappled, depends, first, upon the determination of the general 

 form of the geoid, and second, upon the development of a formula by 

 which the gravitational attractions of adventitious rock masses, and of 

 rock masses varying in density, can be computed. The importance of 

 these inquiries has been recognized by some of the ablest mathemati- 

 cians, physicists, and geologists, including Thomson, Clarke, Pratt, 

 Fisher, Stokes, Helmert, G. IJ. Darwin, Fischer, and others. For some 

 years past Woodward has been engaged upon these problems ; and dur- 

 ing 1888 he published an elaborate discussion of the whole subject, in- 

 cluding analyses of the results reached by former investigators, and 

 formulae applicable in evaluating the deformation of water surfaces by 

 the gravitation.il attraction of ice masses, continents, and mountains, 

 the changes in level of inconstant lakes, etc.* While the immediate 

 result of Woodward's researches can scarcely be regarded as a con- 

 tribution to knowledge of the general phenomena of deformation, his 

 contribution is worthy of note as a sound basis for further investigation. 



DEGRADATION. 



All portions of the earth standing above the level of the sea are sub- 

 ject to degradation. Different processes effect degradation, but incom- 

 paniibly the most potent is the action of rain and rivers, Now the whole 

 subject of hydro-dynamic action has received more attention from Amer- 



*Bnll. of tho U. S, Gool. Survey, No. 48. 



H. Mis. 142 16 



