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; 7. 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As the water was gradually withdrawn, the Loess was deposited farther 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 a!most 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 ot the valleys. 
It is an interesting fact that the Loess is generally separated from the 
