"Again: the rocks lying under the surface are sometimes so full of fissures, that, although they themselves are impervious to water, yet, so completely do these fissures carry off rain, that, in some parts of the county of Durham, they render the sinking of wells useless, and make it necessary for the farmers to drive their cattle many miles for water. It sometimes happens that these fissures, or cracks, penetrate to enormous depths, and are of great width, and filled with sand or clay. These are termed _faults_ by miners; and some, which we lately examined, at distances of from three to four hundred yards from the surface, were from five to fifteen yards in width. These faults, when of clay, are generally the cause of springs appearing at the surface: they arrest the progress of the water in some of the porous strata, and compel it to find an exit, by passing to the surface between the clay and the faces of the ruptured strata. When the fault is of sand or gravel, the opposite effect takes place, if it communicates with any porous stratum; and water, which may have been flowing over the surface, on reaching it, is at once absorbed. In the following diagram, let us suppose that B represents such a clay-fault as has been described, and that A represents a sandy one, and that C and D represent porous strata charged with water. On the water reaching the fault at B, it will be compelled to find its way to the surface--there forming a spring, and rendering the retentive soil, from B to A, wet; but, as soon as it reaches the sandy-fault at A, it is immediately absorbed, and again reaches the porous strata, along which it had traveled before being forced to the surface at B. It will be observed, that the strata at the points of dislocation are not represented as in a line with the portions from which they have been dissevered. This is termed the upthrow of the fault, as at B; and the downthrow, as at A. For the sake of the illustration, the displacement is here shown as very slight; but, in some cases, these elevations and depressions of the strata extend to many hundreds of feet--as, for instance, at the mines of the British Iron Company, at Cefn-Mawre, in North Wales, where the downthrow of the fault is 360 feet. [Illustration: Fig. 7.]