THE EARTH, A FAILING STRUCTURE. 69 
that particular spot, the earthquake center, 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 doubtless extend out for miles 
from the point of fracture, just as vibrations extend over the 
whole earth from an earthquake center. Now it also hap- 
pens that in the lower levels of these deep mines, at a mile 
below the surface of the earth, the solid rock is slowly yield- 
ing, 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 recognized 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 en- 
tered, while the spaces in which they have recently labored 
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 catas- 
trophes which close up any opening suddenly. In this re- 
spect it is an epitome of what is taking place every year in 
the failing earth as it yields under such stresses, due to 
gravity and other causes, as are applied for long periods 
without changes of sign. The solid rock near each large 
opening in the mine responds to every blast in the mine by 
elastic vibrations; and yet, at the same instant, that same 
rock is yielding in a non-elastic manner to the stresses due 
to gravity. 
It, is not necessary to go a mile below the surface of the 
earth to find non-elastic slow yielding. It is commonly 
known to miners working in mines of ordinary depth. 
Each earthquake is evidence that at a particular point or 
* F. W. McNair : Some Problems Connected with Deep Mining in 
the Lake Superior Copper District. Science, January 4, 1907, pp. 
13-18. 
