CRUST AL DENSITIES. 183 



face 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 dry- 

 ing ; 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 inferences 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 prob- 

 ably 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 phenomena of volcanism that their supremacy admits of 

 local exception. 



Nature of density Differences. — If we accept the doctrine 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 den- 

 sity is due merely to a difference in temperature or whether it arises pri- 

 marily 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 statement. 



What caused the continental Plateau ? — The problem of the origin of the 

 continents remains almost untouched. Those who have propounded 

 theories for the formation of mountain ranges have sometimes included 

 continents also, but as a rule without adequate adaptation to the special 

 conditions of the continental problem. So far as I am aware, the subject 

 has been seriously attacked only by our second president, Professor Dana. 

 He postulates a globe with solid nucleus and molten exterior, and pos- 

 tulates, further, local differences of condition, in consequence of which 

 the formation of solid crust on the liquid envelope was for a long period 

 confined to certain districts. In those districts successive crusts were 

 formed, which sunk through the liquid envelope to the solid nucleus, 

 and by their accumulation built up the continental masses. The re- 

 maining areas were afterward consolidated, and sul >s<>< |uent co< >ling shrunk 

 the ocean beds more than it shrunk the continental masses because their 

 initial temperatures (at the beginning of that process) were higher.* 

 That the philosophic mind may find satisfaction in this explanation, it 

 appears necessary to go behind the second postulate and disc-over what 

 were the conditions which determined congelation in certain districts 

 long before it began in others. Can it be shown that the localization of 



* James P. Daua: Manual of Geology, 2d edition, New York, 1874, p. 738. 



