Sculpturing of the Mountains. 307 
We have now some fair idea of what havoc has 
been made of the npheaved plateau in its old age. We 
have traced its checkered history through long cycles 
of change. Once the bed of an ancient sea, then dry 
land covered with a rich vegetation, of which not a 
species has survived, and again depressed beneath the 
waters. All the time fresh layers were being added to 
the load, that was from its sheer weight sinking into 
the earth’s crust. Then the pent-up energy left in the 
globe comes into play, and the sediments are lifted 
high above the sea, and at once the work of destruc- 
tion begins. The separate hills were once a solid 
mass. But Nature sets to work her “agents of 
denudation,” and we have left peaks, crags and cliffs, 
wrecks of the groat plateau that was. 
What are these agents of denudation ? They 
are wind, rain, frost, heat and cold, strean s and 
rivers. If we study any one of these vast valleys, 
it is hard, indeed, to realise that the materials once 
filling the great gorge at our feet were removed by 
running water. Even so distinguished an observer 
as Sir Archibald Geikie confesses that he is taken 
aback at the amount and extent of the work done by 
running water. Writing of the region of extinct 
volcanoes in Auvergne, he tells us : — 
“Standing on the plateau of Pradelle, with its 
remnant of a lava-current, and looking down into the 
valley of Vi liar — a deep gorge, excavated by a rivulet 
through that lava-current, and partially choked up by 
a later coulee of lava which the stream is now wearing 
