hat, 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.]

"Sometimes the strata are disposed in the form of a basin. In this
case, the water percolating through the more elevated ground--near
what may be called the rim--collects in the lower parts of th