310 H. O. WOOD SOME CONSIDERATIONS TOUCHING ON ISOSTASY 



earth, resting upon and confining the potentially yielding matter beneath, 

 and everywhere affected equally by the centrally directed gravity field of 

 the earth, must slump or crush or otherwise fail to sustain and transmit 

 horizontal — that is, tangential — stress, until this has become so great as 

 to bring about failure localized in the weakest elements. When there is 

 deformation of one or more elements, this must, of necessity, involve some 

 change of configuration, molecular if not molar, elsewhere in the outer- 

 most shell, with compensatory readjustments in the subjacent layer capa- 

 ble of ready yielding; for there are no void spaces worth mentioning in 

 the potentially yielding region, and any other means of accommodation 

 of augmented downward pressure in one region means either uplift else- 

 where or further increase in the hydrostatic pressure tending to compact 

 the subjacent material (unless there is storage of energy with endother- 

 mic reactions). If a mountain mass or a continent sinks because it is too 

 heavy to float at the level it has attained, something else must rise or 

 there must be changes of conditions in the depths. 



So, if the earth's radius undergoes shortening throughout a given in- 

 terval, whatever the cause of this (loss of heat, progressive gravitational 

 compacting, crystallization, or reduction of volume with endothermic 

 changes), the peripheral shell must decrease in area. This requires either 

 uniform thickening, or regional distortion and wrinkling, or both thick- 

 ening and distortion together in some combination. Of these possibilities 

 wrinkling is positively observed, though it is not positively proved to be 

 due to this cause. To produce wrinkling under these circumstances, 

 tangential stresses must in some way bring about translation of material 

 toward the wrinkling areas, which will be, at any particular epoch, the 

 weak prisms of the outermost shell. 



Widespread elastic Strain spherically distributed, local Failure 



It is a hypothesis with the writer that accumulating elastic strain in 

 the shell above the depth of incipient yielding, oriented tangentially 

 under the earth's gravity field which keeps the elements of the surface 

 shell in approximate isostatic equilibrium, may ultimately find relief over 

 a very wide region of the globe in the gradual failing of some relatively 

 small region of weakness where upfolding takes place and consequent re- 

 duction of geographic area occurs. But if isostasy is to be maintained 

 approximately at all times, this action must be accompanied both by ad- 

 justment and counter-transfer of matter in the layer of yielding, and by 

 reduction of density in the columns affected by the upfolding, since these 

 can not increase significantly in mass without disturbing isostatic balance. 



