EFFECTS OF MINING. 631 
inconsiderable in extent to deserve notice in a geographical 
point of view.* It is said, however, that in many places in the 
mining regions of England alarming indications of a tendency 
to a wide dislocation of the superficial strata have manifested 
themselves. Indeed, when we consider the measure of the un- 
derground cavities which miners have excavated, we cannot 
but be surprised that grave catastrophes have not often resulted 
from the removal of the foundations on which the crust of our 
earth is laid. The 100,000,000 tons of coal yearly extracted 
from British mines require the withdrawal of subterranean 
strata equal to an area of 20,000 acres one yard deep, or 2,000 
acres ten yards deep. These excavations have gone on for several 
years at this rate, and in smaller proportions for centuries. 
Hence, it cannot be doubted that by these and other like opera- 
tions thee arth has been undermined and honey-combed in many 
countries to an extent that may well excite serious apprehen- 
sions as to the stability of the surface. In any event such ex- 
cavations may interfere materially with the course of subterra- 
nean waters, and it has even been conjectured that the removal 
of large bodies of inetallic ore from their original deposits 
might, at least locally, affect in a sensible degree the magnetic 
and electrical condition cf the earth’s erust.t+ 
* In March, 1873, the imprudent extension of the excavations ina slate mine 
near Morzine, in Savoy, occasioned the fall of a mass of rock measuring more 
than 700,000 yards in cubical contents. A forest of firs was destroyed, and a 
hamlet of twelve houses crushed and buried by the slide. 
+ The exhaustion of the more accessible deposits of coal and other mine- 
rals has compelled the miners in Belgium, England, and other countries, to carry 
their operations to great depths below the surface. At the colliery Des Vi- 
viers, at Cilly near Charleroi, in Belgium, coal is worked at the depth of 2,820 
feet, and one pit has been sunk to the depth of 3,411 feet. It is supposed 
that the internal heat of the earth will render mining impossible below 4,000 
feet. At Clifford Amalgamated Mines, in Cormwall, the temperature at 1,590 
feet stood at 100°, but after the shaft had remained a year open it fell to 83°, 
In another Cornish mine men work at from 110° to 120°, but only twenty min- 
utes at a time, and with cold water thrown frequently over them.—-The lust 
Thirty Yearsin Mining Districts, p. 95. 
Stoppani mentions an abandoned mine at Huttenberg, in Bohemia, of the 
d-pth of 3,775 feet.—Corso di Geologia, i., p. 258, 
