372 TJic American Geologist. June, i90o 
The fluvial hypothesis out of the way. there remain two 
rival theories to account for the Missouri loess, the aeolian 
and the lacustro-marinc. The almost annual occurrence of 
dust storms over the region seems to favor the idea that the 
loess was wind-carried. However, if deposited by precipita- 
tion from a great hight in the air like the fine dirt of these 
"dust-storms," it should be spread more evenly over the sur- 
face, not universally thickening in the valleys. If deposited 
by violent currents of air sweeping close to the surface of the 
earth, it might be Ijlown from the hill-tops and accumulate 
in the valleys, but yet must show more irregularity than the 
loess really does. I imagine the aeolian action would result 
in one of two extremes — the real loess is between them. The 
most pertinent objection to the seolian hypothesis that I can 
raise, is furnished along the southern border of the loess- 
covered country. At Boonville and vicinity, the loess is very 
thick, even thirty or more feet of it capping the highest hills. 
This thick loess extends south about six or eight miles and 
then ends very abruptly, thinning out it is true, but in a com- 
paratively short distance completely disappearing, and south- 
ward from there the only deposits resembling loess are local- 
ized accumulations of undoubted water-laid silt in the valleys. 
This sudden termination of the loess on the south is indicative 
of water rather than of wind action. 
A careful examination of the loess of northern Missouri 
will reveal enough places where its structure may be observed 
to demonstrate that on the uplands as well as in the valleys, 
it is a water-laid deposit. A great lake-like body of water 
covered northern Missouri to the average depth of probably 
several hundred feet. Into this, rivers, resulting from the melt- 
ing of the vast lowan glacier on the north, carried mud which 
spread throughout the lake. Gradually the fine silty and 
clayey particles settled to the bottom to form the loess. There 
was just enough current action to cause this to accumulate 
to a slightly greater thickness in the depressions than over 
the elevations, but not enough to fill the submerged valleys 
and destroy the unevenness of the lake-floor. Along the 
greater depressions, such as that now occupied by the Missis- 
sippi river, and more particularly that of the Alissouri river, 
there was more decided current action (say something like 
