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I 412 STRUCTURAL AND FIELD GEOLOGY 
the Aralo-Caspian depression, the Tundras of Siberia, the 
Llanos and Pampas of South America, etc. 
I Plains of Eroston.—Plains of this class are distinguished 
| by the fact that the surface does not necessarily coincide 
| the Mississippi, and other rivers, the grassy Steppes of Russia, 
i with the underground structure—in the great majority of 
i| cases, indeed, there is no such correspondence. It is only 
when a plain has resulted from the reduction of a series of 
| horizontal strata that external form and internal structure 
Hh can agree. Plains of erosion may be said to represent the 
| final stage of a cycle of erosion—they are the base-levels to 
which old land-surfaces have been reduced. Occurring, as 
they usually do, at low levels, they are lable to become 
covered with alluvial and other deposits, and thus at the 
surface to show as plains of accumulation. Occasionally, 
plains of erosion have been submerged and covered more or 
less deeply with marine deposits. Consequently, when re- 
elevation supervened, the regenerated lands presented the 
appearance of plains of accumulation. It seems not unlikely, 
indeed, that the majority of the latter are merely superimposed 
on pre-existing plains of erosion. The broad, low-lying tracts 
through which the larger rivers of the globe reach the sea 
are probably in many cases plains of erosion more or less 
covered and concealed under alluvial deposits. 
Plateaus or Tablelands.—The term plateau is usually 
applied to any flat land of considerable elevation which is 
separated from contiguous lowlands by somewhat steep slopes. 
It is not always possible, however, to draw a distinction 
between plains and plateaus—for, after all,a piateau is only an 
elevated plain. Standing at a higher level than plains, plateaus 
are necessarily subject to more active and intense erosion, 
and, according to their age, are correspondingly incised and 
denuded. Plateaus of all kinds, as we have learned, tend to 
acquire a mountainous character—to be converted, in short, 
into groups or ranges of relict mountains. 
Plateaus of Accumulation.—These are distinguished by the 
: fact that they are built up of horizontal or approximately 
Bit} horizontal strata; their general or average surface, therefore, 
t) h corresponds with the geological structure. As examples of 
thi plateaus of this type may be cited the Plateau of the 

