which, by further accumulation form rivulets and rivers, and at length terminate in estuaries, where they mix again with their parent ocean. Here they remain, bearing part in all its various functions, until they are again evaporated into the Atmosphere, to pass and repass through the same Cycles of perpetual circulation.
The adaptations of the Atmosphere to this important service in the economy of the Globe belong not to the province of the geologist. Our task is limited to the consideration of the mechanical arrangements in the solid materials of the Earth, by means of which they co-operate with the Atmosphere, in administering to the circulation of the most important of all fluids.
There are two circumstances in the condition of the strata, which exert a material influence in collecting subterraneous stores of water, from which constant supplies are regularly giving forth in the form of springs; the first consists in the Alternation of porous beds of sand and stone, with strata of clay that are impermeable by water;[1] the second circumstance is the Dislocation of these strata, resulting from Fractures and Faults.
The simplest condition under which water is collected within the Earth, is in superficial beds of Gravel which rest on a sub-stratum of any kind of Clay. The Rain that falls upon a bed of gravel sinks down through the interstices of the gravel, and charges its lowest region with a subterraneous sheet of water, which is easily penetrated by wells, that seldom fail except in seasons of extreme drought. The accumulations of this water are relieved by Springs, overflowing from the lower margin of each bed of gravel.
A similar result takes place in almost all kinds of permeable strata, which have beneath them a bed of clay, or of any other impermeable material. The Rain water descends and accumulates in the lower region of each porous stratum next above the clay, and overflows in the same
- ↑ See p. 62.
