THE STRENGTH OF THE EARTH'S CRUST 37 



perpetuate and add to a crust movement initiated by internal 

 causes. Sedimentation is dependent upon the rate and continuity 

 of subsidence as well as upon the rate of deposition. Thus, 

 although the sediments give the most complete record of crustal 

 movements, for the distant past it is not easy to separate cause and 

 effect and ascribe to each its part. Where the thickness of sedi- 

 ments, however, is small, as over much of the continental interior, 

 the cause of submergence is presumably almost wholly independent 

 of the local load. Where the sediments are thick and subsidence 

 rapid, as within the geosynclines, the load imposed by sedimenta- 

 tion may on the contrary become the controlling force. It is a 

 particular phase of deposition, however, which will be considered 

 in this article, a study of the load imposed upon the crust by certain 

 deltas. As Jong as the water plane lies at a constant level the delta 

 builds out at its front. Upon subsidence of the supporting crust 

 the shore retreats inland; less sediment reaches the now sub- 

 merged front, and the delta in consequence grows chiefly by addi- 

 tions to the shoreward part of its upper surface. The two methods 

 of growth not uncommonly alternate upon the same delta, showing 

 the discontinuity of subsidence. In building outward a delta 

 acquires a convex shoreline. This form is clearly related to aggra- 

 dation, not to isostatic uplift, and its volume is a measure of a load 

 inclined to further sinking, the larger rivers tending to drain toward 

 and into the downwarps of a continent. To what degree, then, 

 can a region of the crust which is possibly already resisting down- 

 ward strain bear this added burden ? A preliminary examination 

 will be made of several classes of deltas in order to choose those 

 best adapted to test this question. 



Most of the deltas of Eurasia and South America are at present 

 advancing rapidly into shallow embayments and the faunas of the 

 continental islands show that the latter were recently a part of the 

 land. The physical and organic evidence thus concur in showing 

 that a very recent subsidence has taken place. It is to be con- 

 cluded that a submergent phase in the Cenozoic crustal oscillations 

 has marked the short interval since the last retreat of the Pleisto- 

 cene ice. The great deltas constructed during the late Tertiary 

 and in the Pleistocene are consequently now in great part drowned. 



