222 On the Surface Geology of the Basin of the 



the Carboniferous period — it lias been traversed by a river 

 -which drained the area from which il<>\v the upper Mississippi, 

 the Ohio, the Tennessee, &c. Since the Miocene period, the 

 Missouri, Arkansas, and Red rivers have made their contribu- 

 tions to the flood that flowed through it. The depth to which 

 this channel is cut in the rock proves that at times the river 

 must have flowed at a lower level and with a more rapid cur- 

 rent than now ; while the Tertiary beds formed as high as Iowa 

 and Indiana in this trough, and the more modern Drift clays 

 and boulders which partially All the old rock cuttings, show 

 that the mouth and delta of the river have, in the alternations 

 of continental elevation, travelled up and down the trough 

 at least a thousand miles; and that not only is it true, as assert- 

 ed by Ellet, that every mile between Cairo and New Orleans 

 once held the river's mouth, but that in the several advances 

 and recessions of the waters of the Gulf the mouth has been 

 more than twice at each point. The change of place of the 

 delta has been caused, however, for the most part, by oscilla- 

 tions of the sea level, and not, as Ellet supposed, by the filling 

 of the channel by the materials transported by the river itself. 



Drift Deposi rs. 



The Drift deposits which cover the glacial surface, consisting 

 of fine clays below. Bands and gravel above, large transported 

 boulders on the surface, and the series of lake ridges (beaches) 

 over all, form a sequence of phenomena of which the history 

 is easily read. 



Eri< ( lays. 



Tin- lower series of blue or red clays — -the "Erie clays" 1 of 

 Sir William Logan — over a very large area, Pesl directly OD the 



planed and polished rock-surfaces. These clays are often ac- 

 curately stratified, wen- apparently deposited in deep and 

 generally quiet water, and mark a period when the glacial ice- 



maSSeS, melted by B Change Of climate, reheated northward, 



