Every ton of white salt consumes one ton of rock salt, and a ton of rock
salt represents a solid cubic yard. As 1,200,000 tons of white salt are
made every year at Northwich it follows that at least 1,200,000 cubic
yards of solid foundation are removed from beneath Northwich each year.
This is equal to an annual uniform subsidence of 248 acres one yard
thick. No wonder that Northwich has fits!
Taking the fits as proved, we will now look more closely beneath the
pie-crust of Northwich. The best way to do so is to get into a big tub
which will just hold two people and go down the shaft of a salt mine,
lowered by a windlass. First of all you pass through 32 feet of soil
and drift, and then about 92 feet of what would commonly be called rock.
Then below these 124 feet you come to the first bed of rock salt, which
averages about 75 feet in thickness. Passing through this you come to 30
feet more of rock, and below again is found another bed of rock salt,
which averages in thickness about 90 feet. It is the lower bed of rock
salt which is mined. The bottom of the mine down which I went was 330
feet below the surface, but the atmosphere was delightful, being cool
and dry and not in the least oppressive. A magnificent chamber, 25 feet
high and 17 acres in extent, had been dug out of the salt, and its
extent could easily be gauged by the help of the candles which had been
lit all round the mine. Massive pillars of salt of 10 or 12 feet square
are left at intervals of 25 yards to support the roof.
The rock is got largely by blasting. A hole is drilled, and into the
bottom of the hole a small powder ball is put. Loose powder is placed in
a piece of straw and the straw is lighted. In a few seconds it burns
down to the powder ball, and the rock salt which has lain so quietly in
its bed for aeons breaks up, and in process of time may find itself in
any quarter of the globe.
[Illustration: _T. Birtles, photo._] [_Warrington._