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to explain how it arose. The district, of which North wich 
is the centre, has two beds of rock salt underneath it. The 
first one, about 40 yards from the surface, is on the average 
25 yards thick. Below this there is a bed of much in- 
durated clay about 10 yards thick, and below this again 
the bed of lower rock salt some 35 yards thick. From 1670 
to 1780 all the rock salt mined was obtained from the 
“ Top Rock,” as it is locally called, and the miners left too 
few supporting pillars, and these not large enough, besides 
working the salt out so as to leave only a comparatively 
thin crust of salt as a roof. The great majority of the 
mines in the Top Rock salt have fallen in wholly or 
partially. Since 1780 the rock salt has been “got” from 
the lower bed, and the pillars, especially of late years, have 
been left much larger, the roof at the same time being much 
thicker. Only from 5 to 6 yards of salt near the bottom 
of the bed has been worked. As a rule the mines in the 
bottom rock salt have stood firm, and where the owners 
have worked to their boundaries they have allowed the 
brine to run into the worked-out mines, thus converting 
them into reservoirs. The quantity of rock salt mined is 
small compared with the white salt manufactured. The 
white salt is made from a natural brine which is found on 
the surface of the “Top Rock.” It is found much cheaper to 
let the water do the mining and then pump up the salt in 
solution and drive off the water. The fresh water, as soon 
as it reaches the rock salt, eats it away till it gets fully 
saturated. This water running over the roofs of the old 
“ Top Rock ” mines has in numbers of cases eaten the whole 
of the salt away and opened a communication into the mine 
below. The overlying clays and earths, being deprived 
of their support, fall into the cavity thus opened, and a hole 
is made from the surface. On December 6th this was what 
occurred, and a hole or rift opened right across the course 
of the Wincham Brook, the water immediately rushing 
