44 PHYSIOGRAPHY 



catastrophe. Extensive tunnels at the base of the mountain, for 

 mining, may also have played a part by making the understructure 

 less stable. Many lives were lost, and many buildings destroyed. 

 Instead of sliding down rapidly, surface earths sometimes move 

 down with extreme slowness. This sort of movement is creep. It 



Fig. 29. One type of landslide, near Wardner, Idaho. (From photo, by 



Fairbanks.) 



is often too slow to be seen, but it results in the accumulation of 

 mantle rock, especially earthy matter, at the bases of slopes. Rail- 

 ways at the bases of steep slopes of clayey material are sometimes 

 pushed out by the creep of the clay, and in some places the tracks 

 have to be taken up and laid down anew frequently, especially in 

 wet seasons. 



WEATHERING 



Some of the processes of weathering have already been men- 

 tioned, but it may be added that the chemical changes produced in 

 the rock by the atmosphere, the mechanical changes brought about 



