Great lakes, and the Valley of the Mississippi. 233 



coast present remarkable parallels to them ; and I would sue- 

 gest Poget's Sound, Hood's Canal, and other portions of that 

 wonderful system of navigable channels about Vancouver's 

 Island, as affording interesting and instructive subjects for 

 comparison. Like our lakes, their channels are for the most part 

 excavated from sedimentary strata which form a low and com- 

 paratively level margin to the bases >>\' mountain chains and 

 peaks. They too have their depths and shallows, their basins 

 and bars, and probably all who have seen them will assenl to 

 Prof. Dana's view, that they are the "result of subaerial exca- 

 vation," in which glaciers performed an important part. 



THE "LOESS" OF THE MISSISSIPPI VALLEY. 



The "Bluff formation" of the West, sometimes called 

 "Loess," from its resemblance to the Loess of the Rhine, I 

 have on a preceding page designated as a lacustrine, non-gla- 

 cial Drift deposit. It seems to be the sediment precipitated 

 from the w T aters of our great iidand sea in its shallow and more 

 quiet portions, to which icebergs, with their gravel and boul- 

 ders, had no access, and where the glacial mud was repre* 

 sented only by an impalpable powder, which mingled with the 

 wash of the adjacent land, land shells, &c. 



It is evidently one of the most recent of the deposits which 

 come into the series of Drift phenomena, and was apparently 

 thrown down while the broad water surface which once stretch- 

 ed over the region where it is found was narrowing by drain- 

 age and evaporation, till, by its total disappearance, this sheet 

 of calcareous mud was left. 



It underlie; much of the prairie region, and once filled, often 

 to the brim, the troughs of the Mississippi and Missouri, 

 deeply excavated during the glacial epoch. When the Bystem 

 of drainage was re-established the new rivers began tl 

 vation of their ancient ralleys in the L When they had 



cut into or through this stratum, bo that it stood up in escarp- 

 ments on either side, man came and called it the Bluff for- 



