THIS ROAD WAS RAISED TWENTY FEET IN TWENTY YEARS. NONE OF THESE BUILDINGS IS NOW STANDING
Professor Thompson, in calculating the period of time that was required to lay the salt contents comprised in these deposits, fixed upon an inch in ten years as a fair estimate of the rate of progress at which it was accumulated, and found that it must have taken 21,000 years to lay 60 yds. of rock-salt. With this figure before us, it is interesting to study the following calculation of the salt contents of the Cheshire deposits and of the quantity of mineral that is extracted from the interior of the earth in the form of brine to produce the salt that is made in the Cheshire districts.
Calculating the Northwich salt area at 3 square miles or 1,920 acres or 9,292,800 square yards, and
Taking the upper bed of rock-salt at an average of 25 yds. thick, we have 232,320,000 cubic yds. of rock-salt.
Taking the specific gravity of rock-salt at 2·125, a cubic yard of rock-salt weighs 32 cwts., therefore weight of rock-salt in
upper bed
232,020,000 × 32 tons / 20 = 371,702,000 tons.
Taking the bottom bed as extending over the same area, but having a thickness of 35 yds., we find in it—
9,292,800 × 35 × 32 / 20 = 520,396,800 tons,
or, in both beds together, 892,108,800 tons.