The same is true of the 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 continental and sub-continental material as the ocean bottoms are lower than the continental surfaces. Such static equilibrium, by difference 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 rigidity. It is only the smaller inequalities, such as ridges, peaks, valleys, 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 example, 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, undertaken 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 mountain 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 sinking 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 in 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 wholly 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.