166 CONTINENTAL PROBLEMS OF GEOLOGY. 



about three times as much. The differeuce between the two pressures, 

 or the dilierential pressure, is about 12,000 pounds to the square iuch, 

 aud this force, applied to the entire area of the continental plateau, urges 

 it downward and urges the oceanic plateau upward. Referring again to 

 the diagram in Figure 1, the entire weight of the continental i)lateau, 

 pressing on the track beneath it, tends to produce a transfer of material 

 in the direction from left to right, resulting in the lowering of the higher 

 plateau and the raising of the lower. To the question, how this tendency 

 is counteracted, two general answers have been made: first, that the 

 earth, being solid, by its rigidity maintains its form; second, that the 

 materials of wliich consist the continental plateau and the underlying 

 portions of the lithosphere are, on the whole, lighter than the materials 

 underlying the ocean tioor, and that the difference in density is the 

 couii)leme]it of the difference in volume, so that at some level horizon 

 far below the surface the weights of the superincumbent columns of 

 matter are equal. The first answer regards the horizontal variations of 

 density in the earth's crust as unimportant 5 the second regards them as 

 important. The first maybe called the doctrine of terrestrial rigidity; 

 the second has been called the doctrine of isostasy. At the present time 

 the weight of opinion and, in my judgment, the weight of evidence lie 

 with the doctrine of isostacy. The differential pressure of 12,000 pounds 

 per square inch suffices to crush nearly all rocks, and it may fairly be 

 questioned whether there are any rock masses which in their natural 

 C(mdition near the surfiice of the earth are able to resist it. The samples 

 of rock to which the pressures of the testing machine are applied have 

 been indurated by drying; but it is a fact familiar to quarrymen that 

 rocks in general are softer as they lie in the quarry below the water- 

 line than after they have been exposed to the air and thoroughly dried. 

 It is probable therefore that rocks lying within a few hundred or a few 

 thousand feet of the surface are unable to resist such stresses as are 

 imposed by continents. At greater depths we pass beyond the range 

 of conditions which we can reproduce in our laboratories, and our infer- 

 ences as to physical conditions are less confident. The tendency of 

 subterranean high temperatures is surely to soften all rocks, and the 

 tendency of subterranean high pressures is probably to harden them. 

 It is not known which tendency dominates; but if the tendencies due 

 to pressure are the more powerful, we are at least assured by the phe- 

 nomena of volcanism that their supremacy admits of local exception. 



Nature of drnsity (Jiffereneei'i. — If we accept the (hictrine of isostasy 

 and regard the material under the continents as less dense than that 

 under the ocean floors, the question then arises whether the difference 

 in density is due merely to a difference in temperature or whether it 

 arises primarily from differences in composition. This, which may be 

 called the second problem of the continents, is so intimately related 

 to the one which follows that we may pass it by without fuller state- 

 ment. 



i 



