302 G. R. PUTNAM CONDITION OF THE EARTH'S CRUST 



Among the great unsolved physical problems of the earth's crust are 

 the causes of the subsidence and uplifting of large areas, the overthrust 

 and folding of strata, and the formation of mountain ranges. Although 

 the results of these actions are so openly and impressively displayed in 

 our western mountain regions, the great and long acting forces which 

 accomplished and are still continuing this work are still a subject of dis- 

 cussion with divergent views. Isostasy acts to bring about equilibrium 

 where there is lack of adjustment, and, even without the observational 

 data available, this theory would be a fairly obvious conclusion from evi- 

 dence, within the experience of man, as to the strength of materials and 

 the flow of solids under pressure. The forces which have during ages 

 caused continental areas to oscillate up and down through many thou- 

 sands of feet and have folded and thrust over rock strata of great thick- 

 ness appear to be so far removed from comparisons within human ex- 

 perience as to make a conclusive explanation difficult. 



