I9I2.] STEVENSON— THE FORMATION OF COAL BEDS. 545 



reached almost to the mouth of the Ohio, so that the river bluffs 

 show Tertiary and Quaternary deposits. The broad space, covered 

 with alluvium, extends northward to St. Louis and is continuous 

 thence up the Missouri, which is the main stream, the upper Missis- 

 sippi being only a tributary. This alluvial area, subject to flood 

 prior to construction of levees, is 40 to 70 miles wide from the 

 mouth of the Ohio to the head of the delta, below which it expands 

 to a maximum of about 130 miles. The area contracts above the 

 Ohio, so that, along the Upper Mississippi and the ^lissouri, it 

 frequently is less than 5 miles wide. 



The nature of the river bed varies. From the head of naviga- 

 tion to the mouth of Maria's river, the Missouri, with velocity of 

 2..'] miles per hour, flows on loose gravel ; but farther down in the 

 loess region the bed is shifting silt. In the lower course of the 

 Mississippi, the river has cut its way to the Tertiary beds, which for 

 long distances are swept clear of later sediments by the current. 

 But samples, taken from the bottom at many localities between the 

 Ohio and the Gulf, show that immense deposits of pure silicious 

 sand and fine gravel, wholly free from river mud, exist in the 

 channel-way. These are found below channel-chutes, at all angles 

 to the direction of flow, sometimes even parallel with it; but they 

 rarely extend from one side to the other. The velocity of the water 

 at such places is too great to permit much deposition but is insuffi- 

 cient to wash away the sand. The materials become finer as the 

 Gulf is approached. The bar at the mouth of Southwest pass is 

 of sand and mud, soft mud being inside and around the shoal, 

 while the surface material is much harder, containing much sand. 

 The bar at the mouth of South pass is chiefly sand with spots of 

 soft mud, but away from the shoal, the bottom is covered with soft 

 yellow and blue mud of the passes. 



Studies were made in 185 1 to ascertain whether or not material 

 was pushed along by the river. A keg was laid on the bed in such 

 fashion as to retain suspended matter while permitting unimpeded 

 flow of the water. Coarse sand with some clay was obtained at 

 many localities, while at others only coarse sand remained in the 

 keg. Near the mouth of Red River, at the head of the delta, the 



