MOUNTAINS AND THEIR ORIGIN. 109 
Suppose, for example, that the two upper lay- 
eis of the strata rent apart by the upheaval of 
the mountain are limestone and sandstone, while 
the third is clay and the 
fourth again limestone 
(as in Figure 6). Clay 
is soft, and yields very 
readily to the action of 
rain. In such a valley 
the edges of the strata forming its walls are of 
course exposed, and the clay formation will be 
the first to give way under the action of external 
influences. Gradually the rains wear away its 
substance till it is completely hollowed out. By 
the disintegration of the bed beneath them, the 
lime and sandstone layers above lose their sup- 
port and crumble down, and this process goes on, 
the clay constantly wearing away, and the lime 
and sand above consequently falling in, till the 
upper beds have receded to a great distance, the 
valley has opened to a wide expanse instead of 
being enclosed between two walls, and the lowest 
limestone bed now occupies 
the highest position on the 
mountain. Figure 7 repre- 
sents one of the crests shown 
in Figure 6, after such a level- 
ling process has changed its outline. 
But the phenomena of eruptions in mountain- 
Fig. 7. 
Fig. 6. 
