SURFACE GEOLOGY. 37 



paper on the Drift of the Mississippi valley, published some years since, 

 as a " Lacustrine, non-Glacial Drift deposit," and considered " it the sedi- 

 ment precipitated from the waters of our great inland sea in its shallow 

 and more quiet portions, to which icebergs, with their gravel and bowl- 

 ders, had no access." It is evidently the most recent of our stratified 

 Drift deposits, and I regard it as the equivalent of the lacustrine, ochery 

 clays, enumerated in the preceding sections, and of the surface clay and 

 loam, which overlie the Forest Bed in the Ohio valley. Some years since, 

 Mr. Morris Miller, writing me from Iowa, where he was carefully observ- 

 ing the surface geology, made a suggestion in regard to the origin of the 

 Loess, which has been fully confirmed by the descriptions and conclu- 

 sions of the geologists of Iowa and Missouri ; i. e., that the Loess is the 

 silt brought down by the Missouri river, and spread over the great water 

 basin that occupied the Mississippi valley at the time of its deposition. 

 As the water was gradually withdrawn, the Loess was deposited farther 

 and farther southward, until now it is carried into the Gulf of Mexico. 

 The Loess is simply a river silt, just such as now renders the waters of 

 the Missouri so remarkably turbid. All rivers transport more or less 

 sediment, the quantity and the character of which depend upon the 

 configuration and geology of the country through which they flow. It 

 is said that the sediments of the Amazon render the waters of the At- 

 lantic turbid for 200 or 300 miles from its mouth ; and in the season of 

 floods the Mississippi discolors the waters of the Gulf to an almost equal 

 extent. As the flow of a river current is checked and finally arrested in 

 a body of still water, the sediment it transports is precipitated in the 

 order of its fineness, and the ratio as to quantity of the motion of the 

 water. Hence around the mouth of the river the silt will be coarsest 

 and thickest ; finer and thinner as we recede from that point. In ex- 

 amining the Loess of the Mississippi valley we find that it conforms 

 precisely to this law, being thickest and most sandy about the old mouth 

 of the Missouri in eastern Iowa, Missouri, and western Illinois. North 

 and east of this region the Loess becomes thinner and finer, until it 

 merges into and is lost in the sediments transported by streams which 

 drained into the Mississippi Gulf, or Lake, from the east. 



The Loess is called the Bluff formation, because it once formed the 

 upper part of the filling of the old rocky troughs of the Missouri and 

 Mississippi, and having been but partially washed out by the present 

 streams, often stands as bluffs along the water side. In such cases, how- 

 ever, the Loess is simply a facing to the rocky bluffs which form the true 

 walls of the valleys. 



It is an interesting fact that the Loess is generally separated from the 



