VOLCANOES AND 



At certain times during each day blasts are set off in 

 the solid rock at various places in each mine. Each 

 battery of blasts is a miniature earthquake. In that 

 particular spot, the earthquake centre, the rock is frac- 

 tured within a space limited by a radius of a few feet. 

 Within a large space, limited by a radius of a few 

 hundred feet, elastic vibrations are set up in the solid 

 rock which are sufficiently violent to be perceptible to 

 the touch and to the hearing. Within this larger space 

 no fracture of the rock occurs. Feebler vibrations doubt- 

 less extend out for miles from the point of fracture, just 

 as vibrations extend over the whole earth from an earth- 

 quake centre. Now it also happens that in the lower 

 levels of these deep mines, at a mile below the surface 

 of the earth, the solid rock is slowly yielding, in a non- 

 elastic manner, under the influence of the great weight 

 above it, so that the larger openings are gradually 

 closing up. This is so clearly recognised and progresses 

 so rapidly that it is proposed as routine practice, at the 

 deep levels in these mines, to take out the ore at the 

 distant end of each drift first. The miners will then 

 work back slowly toward the shaft from which the drift 

 is entered, while the spaces in which they have recently 

 laboured gradually close up behind them. The gradual 

 collapse known to be in progress occurs apparently by 

 imperceptible flow and by minor fracturing, but not, 

 as a rule, by catastrophes which close up any opening 

 suddenly. In this respect it is an epitome of what is 

 taking place every year in the failing earth as it yields 

 under such stresses. 



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