190 G. K. GILBERT — CONTINENTAL PROBLEMS. 



continental history characterized in an earlier stage by growth and in a 

 later stage by decadence. In our ignorance of subterranean forces we 

 should use such a priori considerations only as a means for the sugges- 

 tion of hypotheses. As they have doubtless served to promote the theory 

 of continental growth, they should also he permitted to indicate the pos- 

 sibility of continental retrogradation. 



Summary. — The problems of the continents have been touched to-day 

 so briefly that a summary is almost superfluous. The doctrine of isos- 

 tasy, though holding a leading position, has not fully supplanted the 

 doctrine of rigidity. If it be accepted, there remains the question whether 

 heat or composition determines the gravity of the ocean beds and the 

 levity of continents. For the origin of continents we have a single hy- 

 pothesis, which deserves to be more fully compared with the body of 

 modern data. The newly determined configuration of the continental 

 mass has yielded no suggestion as to its origin. The cause of differential 

 elevation and subsidence within the continental plateau is unknown and 

 has probably not been suggested. The permanence of the continental 

 plateau, though highly probable, is not yet fully established ; and the 

 doctrine of continental growth, though generally accepted, has not been 

 placed beyond the field of profitable discussion. Thus the subject of 

 continents affords no less than a half dozen of great problems, whose 

 complete solution belongs to the future. It is not altogether pleasant to 

 deal with a subject in regard to which the domain of our ignorance is so 

 broad ; but if we are optimists we may be comforted by the reflection 

 that the geologists of this generation, at least, will have no occasion, like 

 Alexander, to lament a dearth of worlds to conquer. 



