GEOLOGICAL IMPORTANCE OF SEDIMENTATION 339 



load of sediment is sufficient to lead to isostatic readjustment, or 

 where the river debouches into a natural geosyncline. That such 

 subsidence was common in ancient geosynclines is accepted on the 

 evidence of shallow-water formations accumulated to a thickness of 

 many thousand feet, and whether the formation was made slightly 

 below or above the water surface would apparently have but little' 

 influence upon the movement of subsidence. Again, in periods of 

 quiet the erosion of the lands, as Chamberlin has shown, would tend 

 to lift the level of the sea and lead to an apparent subsidence of all 

 delta regions to the extent of some hundreds of feet from this cause 

 alone. Further, the drainage of the continents is, in general, away 

 from the geanticlinal regions and toward the geosynclinal areas. The 

 land-waste is carried in both directions from the geanticline across 

 the stationary tracts, with the exception of those receiving piedmont 

 deposits, and is thrown down by the river in crossing some region of 

 subsidence, or upon meeting the ocean upon the continental shelf or 

 at the margin of the continental platforms. Usually the waste from 

 at least one side of the geanticline must be carried a considerable dis- 

 tance before passing beyond the limits of the continental platform, 

 and the chances are frequently good for deposition before reaching 

 that limit. 



Where a geosyncline or epicontinental sea is encountered, the sedi- 

 ment will accordingly be concentrated ; and even if the load of waste 

 should not have any influence in leading to isostatic down-sinking, 

 these areas would still be characterized as the great catchment basins 

 of sediment deposited upon the continental platforms ; the most favor- 

 able situation being that illustrated by the Great Valley of California, 

 where a trough exists between mountain ranges, and the waste 

 naturally gravitates into it ; the least favorable being found in such iso- 

 lated basins as those of the Great Lakes, far removed from any consid- 

 erable mountain range. There is therefore frequently, but not neces- 

 sarily a sympathetic relation between regions of subsidence and the 

 discharge of great rivers, with their load of sediment. 



From these lines of reasoning it may be accepted that the portions 

 of the continental platforms occupied by the mouths of the larger 

 rivers have frequently been through past time regions of subsidence, 

 with consequent river aggradation; but the strongest evidence for 



