48 JOSEPH BARRELL 



It appears then that, even allowing for the great increase in tem- 

 perature within the earth's crust at depths greater than can be 

 reached by the limitations of experiment, the demands made upon 

 the strength of the crust by the load of the Niger delta are not 

 greater than can be explained by the theory of the mechanics of 

 materials as now understood. This theory rests, however, even 

 after Adams' experiments, upon only a limited range of laboratory 

 observation, and extending over but hmited periods only, thus 

 demanding extrapolation both of stress and of time when applied 

 to the whole thickness of the outer crust and over hundreds of 

 thousands of years. Therefore the study of the direct evidence 

 supplied by geologic observation is more convincing in regard to 

 the limits of crustal strength. 



These deltas point toward a measure of crustal rigidity capable 

 of sustaining to a large degree the downward strains due to the 

 piling-up and overthrusting of mountains built by tangential 

 forces, or those resulting from the load of sediments in areas of 

 deposition, or those upward strains produced by the erosion of 

 plateaus previously uplifted toward isostatic equilibrium. A 

 final conclusion must, however, await a further discussion in the 

 later parts. 



[To he continued] 



