CONTINENTAL rKOi;i>EMS OF GHOLOCJY. 173 



later stage by deeadence. In our igiioraiicc of subterranean forces we 

 should use sucli a priori coiisidcrations only as a means fortlie sugges- 

 tion of liy[)otliescs. As they liave (h)ubtless served to promote the 

 theory of continental growth, they should also be permitted to indicate 

 the possibility of continental retrogradation. 



Sumnuiri/. — Tiie problems of the continents ha\t' been touciied to-day 

 sobrietly that a snmnuiry is almost superlluous. 'riie doctrine of isos- 

 tasy, though holding a leading i)ositiou, has not full.\- supplanted tlie 

 doctriue of rigidity. If it be accepted, there remains the (juestiou 

 whether heat or composition determines the gra\ity of the ocean beds 

 and the le^'ity of continents. For tlie origin of continents we have a 

 single hyj)othesis, wliich deserves to be more fully compared with the 

 body of modern data. The newly determined configuration of the con- 

 tinental mass has yielded no suggestion as to its oiigin. The cause of 

 differential ele\ ation and sul)sidence within the continental plateau is 

 unknown and has probably not been suggested. The i)ermanence of 

 the continental i)lateau, though highly ])robable, is not yet fully estab- 

 lished; and the doctrine of continental growth, though generally ac- 

 ce]>ted, has not been placed be.\'ond the Held of profitable discussion. 

 Thus the sulyect of continents affords no less than a. half dozen of great 

 problems, whose complete solution belongs to the future, ft is not al- 

 together pleasant to deal with a. subject in regard to which the domain 

 of our ignoran(;e is so broad; but if we are optimists we may be com- 

 forted by the reflectiou that the geologists of this generation, at least, 

 will have no occasion, like Alexander, to lament a dearth of worlds to 

 con(j[uer. 



