Such, nevertheless, is actually the case.  A good-sized Caspian used to spread across the centre of England and north of Ireland in triassic times, bounded here and there, as well as Dr. Hull can make out, by the Welsh Mountains, the Cheviots, and the Donegal Hills, and with the Peak of Derbyshire and the Isle of Man standing out as separate islands from its blue expanse. (We will beg the question that the English seas were then blue.  They are certainly marked so in a very fine cerulean tint on Dr. Hull’s map of Triassic Britain.) Slowly, like most other inland seas, this early British Caspian began to lose weight and to shrivel away to ever smaller dimensions.  In Devonshire, where it appears to have first dried up, we get no salt, but only red marl, with here and there a cubical cast, filling a hole once occupied by rock-salt, though the percolation of the rain has long since melted out that very soluble substance, and replaced it by a mere mould in the characteristic square shape of salt crystals.  But Worcestershire and Cheshire were the seat of the inland sea when it had contracted to the dimensions of a mere salt lake, and begun to throw down its dissolved saline materials.  One of the Cheshire beds is sometimes a hundred feet thick of almost pure and crystalline rock-salt.  The absence of fossils shows that animals must have had as bad a time of it there as in the Dead Sea of our modern Palestine.  The Droitwich brine-pits have been known for many centuries, since they were worked (and taxed) even before the Norman Conquest, as were many other similar wells elsewhere.  But the actual mining of rock-salt as such in England dates back only as far as the reign of King Charles II. of blessed memory, or more definitely to the very year in which the ‘Pilgrim’s Progress’ was conceived and written by John Bunyan.  During that particular summer, an enterprising person at Nantwich had sunk a shaft for coal, which he failed to find; but on his way down he came unexpectedly across the bed of rock-salt, then for the first time discovered as a native mineral.  Since that fortunate accident the beds have been so energetically worked and the springs so energetically pumped that some of the towns built on top of them have got undermined, and now threaten from year to year, in the most literal sense, to cave in.  In fact, one or two subsidences of considerable extent have already taken place, due in part no doubt to the dissolving action of rain water, but in part also to the mode of working.  The mines are approached by a shaft; and, when you get down to the level of the old sea bottom, you find yourself in a sort of artificial gallery, whose roof, with all the world on top of it, is supported every here and there by massive pillars about fifteen feet thick.  Considering that the salt lies often a hundred and fifty yards deep, and that these pillars have to bear the weight of all that depth of solid rock, it is not surprising that subsidences should sometimes occur in abandoned shafts, where the water is allowed to collect, and slowly dissolve away the supporting columns.