SPRINGS are, in almost all cases, the natural overflow or point of escape from some underground reservoir of water. Their classification may be either according to the geological conditions governing the point of location, or according to the chemical com position of its waters. When considerable chemical impurity is present they are usually termed "mineral springs." The rocks which constitute the crust of the earth are either permeable or impermeable to water. During most seasons of the year, in temperate climates, a certain amount of rainfall soaks into any crust formed of permeable strata; the part absorbed may be the complete rainfall during winter months, when the air is saturated with moisture, or may fall to near zero during a dry summer, when all the rainfall is returned to the air as evap oration from the surface. That portion of the rainfall which soaks down below the level of plant-roots goes to replenish the underground reservoir. The shape of the reservoir varies in definitely, according to the geological structure of the area; but it is only the shape of its water surface that concerns springs. This surface, known as the water-table, divides the fully saturated rocks from those which only hold moisture in their minute pores.
In an area of completely permeable rocks the springs issue at or near the valley bottoms ; when, however, the district is made of alternating permeable and impermeable beds, each imperme able bed holds up water on its surface. If the strata are hori zontal, small springs may be found all round the outcrops; but if they are tilted or folded the flow of the underground water will be towards the lowest point on the base of the permeable water-bearing bed. At this point the main spring for that local reservoir will be located.
In strata which are, in the main, impermeable, but somewhat brittle the presence of joints and cracks is of prime importance in determining the direction and amount of flow in the under ground waters. In jointed rocks the rain-water may sink to great depths down one set of joints and rise again along a second, issuing at the surface as a warm or thermal spring; this is the probable cause of the hot springs at Bath. When a permeable bed and an impermeable one are brought into juxtaposition 'through faulting the flow of water in the permeable one is checked ; but since faulting frequently shatters the rock it affords a plane of weakness along which the water will tend to flow. If
the water is flowing under pressure due to an overlying imperme able cover, it may reach the surface as an "artesian spring." The water of artesian springs sometimes carries small particles of solid matter in suspension as well as salts in solution. The solid particles are dropped at the point of exit of the spring and may be cemented by the salts deposited from solution. When this takes place a mound is built up, from the summit of which the spring issues. Hence the "mound springs," such as are seen at their best in the great artesian basin of Australia.
Some of the largest springs issue from thick beds of massive limestone. This type of rock is usually well jointed and, being soluble in rain-water, the joints and marked bedding planes be come enlarged by solution and the whole of the rainfall is ab sorbed in the mass of the rock and flows underground to issue as 'large springs. Frequently these springs yield a somewhat hard water of otherwise great purity.