74 
IOWA ACADEMY OP SCIENCES. 
Number. 
FORMATION. 
Thickness. | 
Depth. 
40-44 
St. Peter 
145 
1100 
36-39 
Upper Oneota 
350 
1350 
33-35 
New Richmond 
60 
1410 
30-31 
Lower Oneota 
22 
1433 
5-29 
St. Croix 
968 
2400 
3-4 
Sioux Quartzite (?) 
10 
2410 
1-2 
Primitive (?) 
20 
2430 
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 PRANK 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, respectively, 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. 
