Fig. 8.
"Again: the higher parts of hilly ground are sometimes composed of very porous and absorbent strata, while the lower portions are more impervious—the soil and subsoil being of a very stiff and retentive description. In this case, the water collected by the porous layers is prevented from finding a ready exit, when it reaches the impervious layers, by the stiff surface-soil. The water is by this means dammed up in some measure, and acquires a considerable degree of pressure; and, forcing itself to the day at various places, it forms those extensive "weeping"-banks which have such an injurious effect upon many of our mountain-pastures. This was the form of spring, or swamp, to the removal of which Elkington principally turned his attention; and the following diagram, taken from a description of his system of draining, will explain the stratification and springs referred to, more clearly.
Fig. 9.
"In some districts, where clay forms the staple of the soil, a bed of sand or gravel, completely saturated with water, occurs at the depth of a few feet from the surface, following all the undulations of the country, and maintaining its position, in relation to the surface, over considerable tracts, here and there pouring forth its waters in a spring, or denoting its proximity, by the subaquatic nature of the herbage. Such a configuration is represented in the following diagram, where A represents the surface-soil; B, the impervious subsoil of clay; C, the bed of sandy-clay or gravel; and D, the lower bed of clay, resting upon the rocky strata beneath.
Fig. 10.
"Springs sometimes communicate with lakes or pools, at higher levels. In such cases, the quantity of water discharged is generally so great, as to form at once a brook or stream of some magnitude. These, therefore, hardly come under the ordinary cognizance of the land-drainer, and are, therefore, here merely referred to."