A CENTURY OF GEOLOGY. 443 



The same is true of tlie large inequalities of surface. Oceanic 

 basins and continental arches must be in static equilibrium or they 

 could not sustain themselves. In order to be in equilibrium the 

 sub-oceanic material must be as much more dense than the conti- 

 nental and sub-continental material as the ocean bottoms are lower 

 than the continental surfaces. Such static equilibrium, by differ- 

 ence of density, is completely explained by the mode of formation 

 of oceanic basins already given. 



So also plateaus and great mountain ranges are at least partly 

 sustained by gravitative equilibrium, but partly also by earth rigid- 

 ity. It is only the smaller inequalities, such as ridges, peaks, val- 

 leys, etc., that are sustained by earth rigidity alone. 



These conclusions are not reached by physical reasonings alone, 

 but are also confirmed by experimental investigations. For ex- 

 ample, a plumb line on the plains of India is deflected indeed 

 toward the Himalayas, as it ought to be, but much less than it 

 would be if the mountain and sub-mountain mass were not less 

 dense and the sub-oceanic material more dense than the average.* 

 Again, gravitative determinations by pendulum oscillations, under- 

 taken by the United States along a line from the Atlantic shore 

 to Salt Lake City, show that the largest inequalities, such as the 

 Appalachian bulge, the Mississippi-basin hollow, and the Rocky 

 Mountain bulge, are in gravitative equilibrium — i. e., the moun- 

 tain and sub-mountain material is as much lighter as the mountain 

 region is higher than the Mississippi-basin region. 



Now, so sensitive is the earth to changes of gravity that, given 

 time enough, it responds to increase or decrease of pressure over 

 large areas by corresponding subsidence or elevation. Hence, all 

 places where great accumulations of sediment are going on are sink- 

 ing under the increased weight, and, contrarily, all places where 

 excessive erosion is going on, as, for example, on high plateaus and 

 great mountain ranges, are rising by relief of pressure. 



This principle of isostasy is undoubtedly a valuable one, which 

 must be borne iii mind in all our reasonings on crust movements, 

 although its importance has been exaggerated by some enthusiastic 

 supporters. Its greatest importance is not as a cause initiating 

 crust movements or determining the features of the earth, but 

 rather as conditioning and modifying the results produced by other 

 causes. The idea belongs Avholly to the latter half of the present 

 century. Commencing about 1840, it has grown in clearness and 

 importance to the present time. 



[To be concluded.^ 



* Pratt, Philosophical Magazine; vol. ix, p. 231, 1855; vol. x, p. 340, 1855; vol. xvi, 

 p. 401, 1858. 



