4i8 



University of California . 



[VOL. I. 



If, as we are compelled to do, we admit that the earth's crust is 

 subject to movements which are not due directly to superficial load- 

 ing or unloading, and suppose that the subsidence in an area 

 receiving sediment is such an independent movement, then the 

 sole condition necessary, to insure the continued laying down 

 of shallow-water deposits, is that the supply of sediment should 

 be locally equal to, or in excess of, the amount necessary to 

 make good the sinking. As the shallow-water sediments, upon 

 which the observations have been made, are off-shore deposits of 

 more or less coarseness, or have been laid down in partially inclosed 

 seas, there is nothing at all improbable in supposing that the 

 sediments were usually furnished in ample abundance to maintain 

 shallow-water conditions. Nor, on the other hand, is this view 

 of subsidence and sedimentation in the least discordant with 

 the known fact that deeper-water conditions, with the formation of 

 massive limestones, have frequently succeeded arenaceous or argilla- 

 ceous sediments, while such a sequence is not so readily accounted 

 for by the ordinary theory of isostasy. The geologist might, with 

 equal reason, argue that the widespread subsidence of the bottom 

 of the Pacific Ocean, postulated by Darwin's theory of coral reefs 

 is a result of the weight of the coral accumulations, inasmuch as 

 the latter have just kept pace with it, as to insist upon isostatic sub- 

 sidence to account for continuous shallow-water conditions of sedi- 

 mentary deposition. The sediments could no more have risen 

 considerably above sea-level than can the coral polyps of the pres- 

 ent day, and that they did not always keep pace with the downward 

 movement is shown whenever a deeper-water deposit succeeds one 

 of shallow-water origin. 



Turning now to what McGee calls the direct data for the theory 

 of isostasy, we are brought to the examples of subsidence presented 

 by our modern areas of deposition, more particularly in the accessi- 

 ble deltas of great rivers. Borings in such deltas have almost 

 invariably shown indubitable evidence of subsidence, but it is to be 

 remarked that this is not necessarily a subsidence of the underlying 

 consolidated strata forming the earth's crust at that point. The 

 work of Professor Hilgard, on the mud-lumps of the Mississippi, 

 shows that at least a portion of such local subsidence may be 



