Vol. XXI, No. 4 



WASHINGTON 



April, 1 910 



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ATIOHAIL 



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LANDSLIDES AND ROCK AVALANCHES 



By Guy Elliott Mitchell 



THE recent disastrous avalanches 

 of snow and earth in the North- 

 west, while of a different charac- 

 ter, recall to mind the tremendous moun- 

 tain slide which destroyed a portion of 

 the town of Frank, Alberta, a few years 

 ago, and also lend interest to geological 

 investigations covering an extensive area 

 in our own San Juan Mountains of Colo- 

 rado, which have been subject to mon- 

 strous rock and landslides, in some in- 

 stances the entire faces of large moun- 

 tains having been demolished. 



It is the younger mountain systems, 

 geologically speaking, which are most 

 subject to these rock avalanches. Thus 

 the Himalayas, which represent but in- 

 fant industries, though lusty ones, in the 

 mountain building line, have a way, like 

 other youngsters of immature character, 

 of tumbling about in a wholesale fashion 

 which would result in great catastrophes 

 were their slopes and valleys populated 

 to any great extent. 



Sir William Conway describes the 

 matter of a little shifting of rock which 

 caused the formation of Gohna Lake, in 

 the Central Himalayas, where the spur 

 of a large mountain mass pitched bodily 

 into the valle\' below. The front of the 

 mountain had been undermined by 

 springs until there was no longer suffi- 



cient support, and in the twinkling of an 

 eye a large part of the mountain slid 

 down and shot across the valley, dam- 

 ming its river'with a lofty and imper- 

 vious wall. Masses of rock were hurled 

 a mile azvay, blocks of limestone iveigh- 

 iiit:; 30 to 50 tons being sent through the 

 air like huge cannon shots. It is esti- 

 mated that this slide carried with it 

 800,000,000 tons of rock and debris. 



Plenty of Himalayan landslips quite 

 as extensive as this have been recorded 

 in the last half century, while among 

 the remote and uninhabited regions of 

 the great ranges numbers more are of 

 constant occurrence. 



The formations of the San Juan 

 Mountain landslide area point to many 

 such slides as these having occurred. 

 Fortunately this catastrophe era has 

 ended for the mountains of the Ignited 

 States, although it is true that some 

 movement is still in progress, and as in 

 the Alps and in Alberta, man's mining 

 operations may precipitate disasters. 



CAUSE OF LANDSLIDES 



Aside from the- study of landslides 

 with reference to the safety of human 

 life, there is economic value in their in- 

 vestigation as bearing upon man's search 

 for the precious metals. The geologist 



