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384 _ “DYNAMICAL GEOLOGY. ——_‘[Boox IIT. | 
been entirely cut out of older alluvium (in which case, of course, 
the valleys must have been as deep as now before the forma+ 
tion of the terraces); whether they afford any indications of 
having been formed during a period of greater rainfall, when the 
rivers were larger than at present; whether they point to upheaval 
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Fig. 121—OxLp TrRRAcES oN THE LEFT BANK OF THE YELLOWSTONE RIVER, 
ABOVE THE First CaNon. MonrTana, 
of the interior of the country which would accelerate the erosive 
action of the streams, or to depression of the interior or rise 
of the seaward tracts, which would diminish that action and 
increase the deposition of alluvium. Professor Dana has con- 
nected the terraces of America with the elevation of the axis of that 
continent. | | | 
There can be no doubt that both in Europe and North America 
the rivers at a comparatively recent geological period had a much 
greater volume than they now possess. Their valleys are not only 
marked by terraces but in many cases are filled with the deep and 
extensive deposit known as loess. The Rhine and the Danube are 
both fringed for long distances by high banks composed of this 
deposit. Still more extensive is the loess of the Mississippi basin; it 
extends for hundreds of miles along the river, forming bluffs, which 
rise 150 feet or more above the present valley bottom. lLoessisa pale 
yellow, calcareous, friable clay, extremely fine in texture, with little or 
no trace of stratification. It contains land and fresh-water shells 
with bones of land animals and remains of land vegetation. It has 
been generally supposed to have been laid down by the rivers during 
a period when they were swollen with muddy water derived from 
copious rains and melting snows. It seems, however, to shade off 
laterally into loess which, stretching far beyond any conceivable 
overflow of the rivers, must be due either to rain-wash or to tha 
sand-drift already described (p. 3822). . Be 
