TEE STRENGTH OF THE EARTH'S CRUST 45 



to the continents. Parallel to them the sea has great fore-deeps. 

 It appears as though the mountain ranges had been piled too high 

 by tangential forces, and, by virtue of the partial rigidity of the 

 crust, had depressed the neighboring ocean bottoms. Erosion of 

 the coastal mountains and deposition of their waste in the fore- 

 deep would tend, up to a certain limit, to equalize the strain in the 

 crust. In that case it might happen that, although the mass of 

 the delta measures a stress, this might be opposite in character 

 to pre-existing stresses, with the result that the strain upon the 

 crust beneath the delta before the infilling might be as great or 

 greater, but in an opposite direction. The greatest remaining 

 strain within the sea-bottom could conceivably be an upward 

 strain under the parts of the fore-deep not filled. 



Such relations are not found around abyssal slopes of the 

 Atlantic type. These are regarded by many geologists following 

 the lead of Suess as made by marginal downbreaking of the con- 

 tinents. They have but little or no relation to the older folded 

 structures and no excessive deeps parallel to the continental mar- 

 gins. If these relations of the Atlantic and Indian oceans to the 

 continents are rightly interpreted as to cause, it is probable that 

 the stresses which make for downsinking extend beyond the parts 

 already foundered. The margin of continents and ocean basins 

 are not likely to be depressed too low, but if remaining out of 

 isostatic adjustment they would tend rather to stand too high. 

 There is no theoretic reason to believe, therefore, that the Nile and 

 Niger deltas have neutralized pre-existing strains. They are best 

 regarded as real and present burdens sustained by the rigidity of 

 the crust. 



Whether or not, however, the building of deltas produced 

 stresses of a character identical with, or opposite to, those previously 

 existing in the region, the stress gradient between the areas of the 

 delta and the surrounding areas would be measured by the weight 

 of the sediments, and this would tend to produce differential 

 flexure. It would seem to be a logical conclusion, therefore, from 

 these tests, that certain parts of the earth's outer crust can resist 

 for considerable periods of time vertical stresses at least equivalent 

 to the weight in air of 10,000-25,000 cubic miles of rock in lenslike 



