It is not the presence of water over the beds of salt or in the old salt workings which causes the damage, because when such water has taken up salt to the extent of a fourth of its bulk, it remains inactive and makes no further ravages upon the mineral earths with which it is in contact. But when the saturated brine is pumped up and its place is taken with a new supply of water which collects its tribute from the salt strata, and that water, in its turn, is raised, to be replaced by more, and when it is known that each 100 tons of water that traverses the salt-bed to the pumps carries away with it 25 tons of solid earth, the work of destruction that is continually going on is explained. It may be convenient to explain at this point that the subsidences caused by this simple operation of removing rock-salt from the earth in the form of brine are divided into three classes, viz.-- 1. Shallow troughs, with sides not terraced or broken up. 2. Very shallow depressions extending over considerable areas. 3. Deep troughs, much broken up, and with stepped or terraced sides. With these three classes in mind, it is easy to follow the results of the action of the subterranean brine and associate the causes with the effects produced. At first the water flowed over the salt in irregular channels and reached the pumping centres by devious routes, but after a time it made defined courses for itself exactly as the rainfall carves out for itself channels on the surface of the earth. These underground streams of brine all gravitate towards the pumps, widening and deepening as the continually renewed water takes up its supply of salt. Where the earths overlying these brine “runs” are not too tenacious, they soon follow the hollow or trough formed on the surface of the salt bed, and a corresponding hollow or trough is formed on the surface of the ground. Where the hollow forms at an early stage, it rarely attains any considerable depth, for the sinking earths impede the course of the flowing brine stream and cause the fluid to spread and be diffused over a wider area. These subsidences are the shallow troughs, not stepped or terraced on the sides, and are best seen in streets and roads where the weight of the houses and the constant passage of traffic cause the earths to gradually follow the wasting surface of the salt. Where, at a considerable distance from the shafts, the water has not formed for itself a definite channel, it percolates over a wide area. The denudation in such cases is more generally spread, and a very extensive shallow trough or basin is formed. Again, where the pumping stations are close together, or in the same line, the various rivulets or streams of brine converge into one broad and deep channel, in which the denudation proceeds with great rapidity. The magnitude of these channels causes the super-incumbent ground to subside swiftly, forming deep troughs with stepped or terraced sides, where the earths have broken away in huge masses.