LOWER RAPIDS OF THE MISSISSIPPI 7 



encroachment of the Labrador ice-field. It is reasonable to 

 suppose that the deflection caused by the Kewatin ice-field 

 might give the stream a course farther to the east than the lower 

 rapids, since the region across which the rapids have been opened 

 appears to have been entirely covered by the Kewatin ice-field 

 at each of its invasions. It will be necessary, therefore, to 

 determine whether the Kewatin field did not establish the Mis- 

 sissippi in a course east of the rapids, and whether that course 

 was not held by the Mississippi until the Labrador ice-field 

 forced it westward into its present course across the lower rapids. 



Turning now to the question of the influence of the supposed 

 earlier invasion of the Kewatin ice-field, a few remarks seem 

 necessary concerning the deposits made by that ice-field. The 

 lowest conspicuous member of the drift series in eastern Iowa is 

 a sheet of dark blue till, often nearly black, which is thickly set 

 with fragments of wood and coal. This is overlain by a sheet 

 of blue-gray till, which differs from the blue-black till in texture 

 and rock constituents, as well as in color. It shows a decided 

 tendency to break into rectangular blocks and often presents 

 vertical fissures, extending to a depth of many feet, which are 

 filled with sand and deeply oxidized clay. The blue-black till 

 is very friable and seldom shows a tendency to break into rectan- 

 gular blocks, while the few fissures which it contains traverse it 

 in oblique rather than vertical lines. The blue-gray till carries 

 much less vegetal material and coal fragments than the blue- 

 black till. It differs also from the blue-black till in containing 

 a larger percentage of greenstone rocks. These differences have 

 naturally led to the suspicion that two quite distinct sheets of 

 till are present, and this suspicion is confirmed by the occasional 

 occurrence of a black soil at the surface of the blue-black till. 

 Such exposures are rare compared with those of the Yarmouth 

 soil, found between the Kansan and Illinoian till sheets, 1 but 

 their rare occurrence may not demonstrate that the interval of 

 deglaciation is of minor importance. From conversations with 

 Calvin, Norton, and Bain, I am led to think that a large part of 



'See Jour. Geol., Vol. VI, 1898, pp. 81-85. 



