, sand, or
chalk, permitting a free passage to water.

[Illustration: Fig. 5.]

"When rain falls in such a district, after sinking through the
surface-layer (represented in the diagram by a narrow band), it
reaches the stratified layers beneath. Through these it still
further sinks, if they are porous, until it reaches some impervious
stratum, which arrests its directly-downward course, and compels it
to find its way along its upper surface. Thus, the rain which falls
on the space represented between B and D, is compelled, by the
impervious strata, to flow towards C. Here it is at once absorbed,
but is again immediately arrested by the impervious layer E; it is,
therefore, compelled to pass through the porous stratum C, along
the surface of E to A, where it pours forth in a fountain, or forms
a morass or swamp, proportionate in size or extent to the tract of
country between B and D, or the quantity of rain which falls upon
it. In such a case as is here represented, it will be obvious that
the spring may often be at a great distance from the district from
which it derives its supplies; and this accounts for the fact, that
drainage-works on a large scale sometimes materially lessen the
supply of water at places remote from the scene of operations.

"In the instance given above, the water forming the spring is
represented as gaining access to the porous stratum, at a point
where it crops out from beneath an impervious one, and as passing
along to its point of discharge at a considerable depth, and under
several layers of various characters. Sometimes, in an undulating
country, large tracts may rest immediately upon some highly-porous
stratum--as from B to C, in the following diagram--rendering the
necessity for draining less apparent; while the country from A to
B, and from C to D, may be full of springs and marshes--arising,
partly, from the rain itself, which falls in these latter
districts, being unable to find a way of escape, and partly from
the natural drainage of the more porous soils adjoining being
discharged upon it.

[Illustration: Fig. 6.]

"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, t