GEOLOGICAL IMPORTANCE OF SEDIMENTATION 335 



sea. The ability of large rivers to maintain land surfaces over sinking 

 areas may be illustrated by taking a hypothetical case. Choosing 

 Lake Superior, since it is the largest body of fresh water in the world, 

 and the Mississippi-Missouri river system as a type of the greater 

 rivers of the globe flowing from regions of rapid erosion, it may be 

 computed how long it would take the Mississippi to fill the Lake 

 Superior basin with sediment. Taking the annual discharge of the 

 Mississippi, according to Humphrey and Abbot, as a mass of sedi- 

 ment sufficient to cover one square mile 268 feet deep, and again 

 taking the area of Lake Superior as 32,000 square miles, and the 

 average depth, derived from the contours of the bottom, as 550 feet, 

 it may be readily computed that the Mississippi would fill up the basin 

 in approximately 66,000 years. Therefore, if such a basin should 

 originate in the path of a great river bearing a quantity of sediment 

 equal to that of the Mississippi, it would only show at the surface 

 during its subsidence as a somewhat swampy alluvial plain, without 

 a distinct lacustrine stage, unless the movement of subsidence was 

 irregular or the entire depression originated in less than 66,000 years. 

 It is not probable that the majority of epicontinental basins originate 

 as rapidly as this, indicating the conclusion that in periods of high 

 land relief and rapid erosion river deposits, rather than those of lacus- 

 trine or marine origin, may be expected to fill the down-warpings 

 within continental areas, and to a lesser extent those which are mar- 

 ginal. On the other hand, Forshey has computed that it would take 

 the Mississippi 11,000,000 years to fill the Gulf of Mexico with sedi- 

 ment, providing that the bottom did not sink under the load; and 

 this points the contrast between the relatively small and shallow epi- 

 continental basins and the true oceanic gulfs or mediterranean seas. 

 In conclusion, it is seen, then, that interior basins may be divided 

 into two classes, the shallow and the deep. The former, if within 

 the reach of important rivers, may be maintained as a continual land 

 surface, the basin nature being not conspicuous ; the latter will more 

 usually form true mediterranean seas. Basins of these two classes 

 occupy appreciable portions of the present continental surfaces, and 

 doubtless have frequently been as important in the past. The shal- 

 low warpings still belong to the continental platforms, and their 

 deposits, either fluviatile, lacustrine, or marine, are frequently exposed 



