“The second and tertiary geological formations, such as those underneath London and Paris, often present the appearance of immense basins; the boundary or rim of the basin having been formed by an upheaval of the subjacent strata. In these formations it often happens that a porous stratum, consisting of sand, sandstone, chalk, and other calcareous matter, is included between two impermeable layers of clay, so as to form a flat, porous ‘U’ tube, continuous from side to side of the valley, the outcrop on the surrounding hills forming the mouth of the tube. The rain filtering down the porous layer to the bottom of the basin, forms there a subterranean pore, which, with the liquid or semi-liquid column pressing upon it, constitutes a sort of huge natural hydrostatic bellows; sometimes the pressure on the superincumbent crust is so great as to cause an upheaval or disturbance of the valley, and there can be little doubt that many earthquakes that are manifestly not of volcanic origin, are due to this simple cause.”--(Ninth edition Encyclopedia Britannica.) “An overflow results only when the surface that supplies the water-bearing stratum is at an elevation superior to the surface of the ground where the well is located, and the water-bearing stratum is confined between impervious strata. In such cases the hydrostatic pressure from the higher source forces the water up to the mouth of the bore.”--(Fanning Water Supply.) “In the tertiary formations the porous layers are not so thick as in the secondary, and, consequently, the occurrence of underground lakes is not on so grand a scale; but there being more frequent attenuation of these sandy beds, we find a greater number of them, and often a series of natural fountains may be obtained in the same valley preceding from water-bearing strata at different depths, and rising to different heights. “It does not follow that all the essentials for an Artesian well are present, though two impermeable strata, with a porous one between, may crop out around a basin. There must be, in the first place, continuity of the permeable bed for the uninterrupted passage of the water, and there must be, on the other hand, no flaw or breach in either of the confining layers by which the water might escape. To one or the other of the causes is due the failure of many attempts to find Artesian wells, where, from appearances, they might be expected. It has occasionally happened that on deepening the bore, with the hope of increasing the flow of water, it has ceased altogether, doubtless from the lower confining layer being pierced, and the water allowed to escape by another outlet. “The subterranean bore is frequently of small extent, and of the nature of a channel rather than of a broad sheet of water; and the existence of one spring is no guarantee that another will be found by merely boring to the same depth in the neighborhood.