CHAPTER XXV 
GEOLOGICAL STRUCTURE AND SURFACE FEATURES 
Denudation and the Evolution of Surface Features. Mountains classified 
according to Structure and Origin ; Original or Tectonic Mountains 
—their Erosion and Transformation ; Subsequent or Relict Moun- 
tains. Plains and Plateaus of Accumulation and Erosion. Original 
or Tectonic and Subsequent or Erosion Valleys. Basins. 
THE student of structural geology soon learns to recognise 
the importance of denudation. Almost everywhere he is 
confronted with evidence to show that enormous masses of 
rock have been removed from the land. He is prepared, 
therefore, to believe that erosion must have played an 
important part in the production of surface features. As his 
observations extend, he begins to realise that the configura- 
tion or shape of the land largely depends upon the lithological 
character and the geological structure of the underlying rock- 
masses. So constantly does this remarkable relationship 
appear, that it cannot be merely accidental. In regions which 
have been highly disturbed within late geological times, it 
is doubtless true that the larger or more prominent features 
of the land are the direct result of deformation. We see 
certain great mountain chains, the geological structure of 
which compels us to believe that they are simply wrinkles 
or flexures of the earth’s crust. But in other lands which have 
not been subject to crustal deformation for a prolonged period of 
time, we can seldom trace any coincidence between surface 
features and underground structures. The mountains do not 
correspond with anticlinal folds—the valleys do not lie in 
synclinal troughs. The relation between the configuration of 
the ground and its internal structure is therefore not direct. 
In a word, observation has conclusively shown that in all 
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