IOWA ACADEMY OF SCIENCES. 



In the discussion of this paper Mr. Leverett called attention 

 to the need of careful examination of the suposed Sioux quart- 

 zite drillings, especially since a well at the neighboring town 

 of Aledo, 111 , reached a depth of 3,100 feet without touching 

 the quartzite. 



THE LOWER RAPIDS OF THE MISSISSIPPI RIVER. 



BY FRANK LEVERETT. 



INTRODUCTORY. 



In the early days of navigation on the Mississippi, two 

 important rapids were found to interrupt the passage of ves- 

 sels at low water stages; one, about fifteen miles in length, 

 being above the city of Rock Island, 111., and the other, about 

 eleven miles in length, above the city of Keokuk, Iowa. These 

 became known, resj^ectively, as the upper and lower rapids 

 The latter are also called the Des Moines rapids because of the 

 situation above the mouth of the Des Moines river. 



In both rapids the obstructions consist of rock ledges, yet 

 the form of arrangement of the ledges is not the same. The 

 upper rapids consist of a succession of rock barriers called 

 "chains," each usually but a fraction of a mile in breadth, 

 which pass across the river channel and are separated by pools 

 or stretches of slack water. The lower rapids are more uni- 

 form, there being a nearly continuous descent across them. 

 The rate of descent, however, varies, as shown below. In open- 



*Published by permission of the director of the United States Geological Survey. 



