Scientific exploration work and a great number of borings have enabled us to form a fairly accurate estimate of the area of the Cheshire salt-beds, except in the region to the north of the deposits, where systematic examination has still to be undertaken. Without quoting the exact locations of bore-holes and distances between them--particulars which would convey little or nothing to the general reader--it may be broadly stated that the proved salt area in the Northwich district is about four square miles, while the increasing quantity of marl that is mixed with the salt to the northward favours the probability that the beds soon die out in that direction. The Winsford salt district comprises an area of six square miles, while it is calculated, with less preciseness, that the Middlewich, Nantwich, and Lawton districts all contain large quantities of rock-salt. At the bore-hole at Marston, which appears to be on the highest proved portion of the salt-bed, the salt is found at 47 ft. below ordnance datum, and from this central point the surface of the salt falls away gently in every direction. Mr. James Thompson, a recognized local authority upon salt and salt-mining, writing on the subject nearly fifty years ago, gave the thickness of the upper bed of rock-salt at about twenty-five yards, but that thickness was only maintained within a circle of about three miles in circumference, beyond which he found that it thinned off rapidly on the upper surface. The extent of the second or bottom bed, from which all the rock-salt produced in Cheshire since 1780 has been extracted, is less clearly defined, but it is known to underlie not only the whole of the upper bed, but a further considerable area in all directions. [Illustration: 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--