MISSISSIPPI RIVER IMPROVEMENT. 



605 



culties if erosion became necessary. In the 

 northern portions of the valley it consists of 

 clay and sand with beds of lignite; in the 

 southern, of greensand, marls, and marly clay. 



Below Cairo the alluvium of the Mississippi 

 occupies a huge trough carved out of the soft 

 eocene tertiary layers, bounded by loess-capped 

 bluffs. Westward of this ridge, between Com- 

 merce, Missouri, and Helena, Arkansas, is an- 

 other alluvial region, nearly as extensive as 

 that of the Mississippi, whose basin, drained by 

 insignificant streams, was probably excavated 

 by floods caused by erosion of the present val- 

 ley of the Mississippi. This alluvium consists 

 of clays, sands, and gravel largely siliceous, the 

 material generally growing coarser as it de- 

 scends below the surface. Deeply seated ex- 

 tensive clay layers are rare and wanting in 

 traces of vegetation. Remains of trees in a 

 vertical position are sometimes seen in a cut- 

 ting bank, but not lower than depressions 

 in the bottom-lands where forests are now 

 growing, remote from the channel. Logs in 

 good preservation are found deeply imbedded 

 in sand or gravel, but their position and the 

 absence of bark show that they are drifts. 

 Borings during 1879 revealed the general law 

 that the nearer the Gulf the deeper the sur- 

 face-silt deposits, and the greater the surface 

 changes from fluviatile forces. The clay beds 

 are sometimes entirely wanting, and some- 

 times, as at the foot of cut-off bends, they 

 may be of great thickness, the deep trough 

 having been slowly filled with the finer silt. 

 At Lake Providence a depth of one hundred 

 and nine feet of fine clay and sand is sepa- 

 rated from the lignite by a bed of unusually 

 heavy gravel. 



There is a decrease in the size of the gravel 

 as the river is descended, especially below 

 Lake Providence. The gravel layers are of 

 identical material above the mouth of the Ar- 

 kansas ; below it, the larger pebbles are of sili- 

 ceous clay, stones, and of white cherty ma- 

 terial. Red and yellow jasper, brown-stone, 

 chert, clay-balls, and water-worn lignite com- 

 pose the gravel-beds. The sand, which is usually 

 found between the surface clays and the gravel 

 layers, is always siliceous, and generally wa- 

 ter-worn. Mica and the whitish-gray sand of 

 the northern lignitic group are lacking in the 

 alluvial strata. Crystals of iron pyrites are 

 sometimes found in the gravel-beds. Water 

 from wells throughout this region is strongly 

 impregnated with iron, and unpalatable. Wa- 

 ter nearer the surface is preferred, being clear 

 and agreeable ; but it is saturated with vegeta- 

 ble impurities, and productive of malarial dis- 

 orders. So far, no wells have been sunk 

 through the alluvium into the older strata. 



The bluffs on either side of the Mississippi below 

 Cairo are composed at top of the quaternary loess, 

 resting upon the orange sand, or bluff gravel { which 

 in turn rests upon the tertiary layers. The thickness 

 of the loess varies continually, depending upon the 

 elevation of the orange sand as left by the glacial 

 floods. The thickness of the orange-sand deposit, 



even at points nearly adjacent, also varies prcatlv, 

 and depends not only upon the height at which its 



had been at some point deeply furrowed by the glacial 

 waters, whose central current plowed out the huge 

 alluvial trough of the Mississippi. 



All the borings between Cairo and the Gulf 

 show that the alluvium rests on tertiary de- 

 posits. Back of Helena were found the first 

 marine tertiary strata which prevail below He- 

 lena as the underlying formation. High banks, 

 far from marking tertiary deposits, cover the 

 deepest alluvial beds of the Mississippi bottoms. 

 Vegetation imbedded in the clays near the sur- 

 face proves that they were not built up, as is 

 the alluvion, and afterward elevated by earth- 

 quake-action. To some water agency mightier 

 than the modern Mississippi must this forma- 

 tion be attributed. Professor Potter supposes 

 that these high prairies are the sand-bars of the 

 glacial stream. The small size of the fossil 

 shells indicates the periodic destruction of the 

 salt-water tribes by northern fresh waters an- 

 terior to this glacial flood. 



Southward of Gainesville there are beds of 

 white quartzite. This extraordinary Silurian 

 outcrop is doubtless due to the induration of the 

 white tertiary sand, like that shown in the 

 borings near New Madrid and Plum Point. 



In these two vicinities the bed material 

 which' offered the most resistance to boring- 

 tools is lignite. Below Memphis are beds of 

 hard clay, popularly known as soap-stone, which 

 were reached at ninety feet beneath the high- 

 water mark of 1880. At Helena and Choctaw 

 Bar there are stiff clays, probably of concretion- 

 ary origin. Near Greenville, sub-alluvial layers 

 were found at 88-4 feet, the least depth below 

 the surface anywhere obtained. Save in one 

 or two instances, nothing was discovered in 

 the alluvial layers beneath the surface clays 

 which would offer any. decided resistance to 

 erosion. There is no difficulty in working in 

 material such as this, were it judged proper to 

 deepen the bed of the river. In an engineer- 

 ing point of view this conclusion is important. 

 It also affects the question of levees. Unnec- 

 essary heights have been proposed for them, 

 on the assumption that there is an immovable 

 river-bed. Their estimated cost, made on these 

 grounds, can certainly be diminished. 



The commission are not in possession of the data 

 deemed necessary in making a trustworthy estimate of 

 the entire cost or a levee system, but a great reduction 

 is evidently practicable in the amount of former esti- 

 mates based on the assumed existence of a river-bed not 

 subject to erosion, and on levee-work at forty or fifty 

 cents per cubic yard. 



The discussion establishes a strong presumption 

 that the levees of the Mississippi do not now stand as 

 works superimposed on the normal banks to restrain 

 additional flood-volume, but as artificial barriers to 

 replace that natural height of bank which has been 

 lost from causes connected with the occupation and 

 cultivation, not only of the banks of the main stream, 

 but also of the basins drained by the tributaries. 

 Their present function is not so much to increase 

 the flood-volume abnormally as to maintain that part 



