Where the earth consists of strong marls and a kind of flagstone they are very tenacious and remain suspended for a considerable time over these deeper cavities. When they will bear no longer, a sudden fall occurs in one spot, and tens of thousands of tons of suspended earths fall into the trough below, forcing out the stream of brine at the weaker places and leaving a huge, crater-shaped hole on the surface, which fills with water. [Illustration: REMARKABLE SUBSIDENCE IN NORTHWICH] In addition to the three classes of subsidences already mentioned, there is another which is the result of a combination of collapses of the surface earth caused by the rock-salt mining operations, and the denudation of subterranean strata caused by the pumping of brine. The pumping from the reservoirs formed by the flooding of the old mines does not empty these huge receptacles, as the place of the brine is continuously retaken by fresh water, which naturally gravitates to these centres and proceeds to dissolve and take up its quota of rock-salt. When a subsidence occurs on the site of these old workings it is of the most destructive nature, and as all the top-rock mines were in the neighbourhood of streams and brooks, the surface waters flow into the cavity until it is filled to the level of the earth and allows the streams to pursue their proper course. But as fast as the fresh water becomes saturated and is pumped to the surface, the overlying stream or brook lets in further supplies of fresh water to fill the vacuum, and the work of internal destruction is followed by further subsidences of the suspended earths.