The following table sets forth the principal characteristice of these water-besring strata:— In endeavouring to ascertain the qualities of the underground waters derived from different forma tions, it may he generally assumed that those drawn from limestone formations are " hard," and those from sandstone " soft." ONNing, however, to variations in the nature of some of the strata in different localities, and to the greater or 1. ss proportion of carbonate of lime, carbonate of magnesia, salts of iron, &c., which they contain, the quality of the water from the same formation is liable to variation according to locality. Although this subject has been fully dealt with by various authors, it may be well to give here a, brief summary of the results, as far as they have been ascertained, in different localities.
(a) Water from the Chalk.—The percolation of the rain through this formation, amounting in proportion to about one-third of the actual rainfall, is so exceedingly slow, that the water has abundant time to take up a large proportion of carbonate of lime from the rock itself, hence chalk water is naturally hard. It seems, from observations made on the chalk hills, that it takes ii.om four to six months for the rain to reaqh a depth of 200 to 300 ft., so that the water which is dmwn from this depth in summer belongs to the rainfall of the preceding winter. The total quantity of solid matter in chalk water varies from 31 to 32-5 in 100,000 parts, of which 16-4 to 21 parts are carbonate of lime. In the case of large works, this mineral ingredient can he dealt with by Clark's eoftening process ; but for country villages there seems to be no plan of easy applicatian for lessening the amount of calcareous matter, except that of boiling, by which the hardness is reduced from 241 to 3.7 in extreme cases. Chalk water, though hard, is very suitable for many purposes, especially for the important one of brewing.
(b) Upper Greensand.—The water from the upper Greensand, which immediately underlies the Chalk, is probably a little less hard than that from the Chalk itself.
(c) Lower Greensand.—The water from this formation, which is separated from the upper Greensand and the Chalk by an impermeable stratum known as the Gault, is remarkably pure, and decidedly " soft." Samples taken from five localities gave a mean result of 7-9 of solid matter in 100,000 parts of water. Water obtained from this source is therefore very suitablo for drinking purposes. As the sands are generally loose and incoherent, they absorb nearly Bll the rain which falls on their surface, except that given off by evaporation or imbibed by vegetation.
(d) Oolite Limestones.—The water from these formations, which are much interstratified with sand-beds, is more or less hard, yet less so than that from the Chalk. Of the proportion of solid matter in the waters of the Oolites, that found in the fine springs of South Cerney, near Ciren cester, which rise along the line of a large fault, may be taken as a sample. The total amount of solid matter was fuund to be 18 grains per gallon, of which 1.25 grain was of organic cnigin. The water from the Severn springs near Cheltenham, from the inferior Oolite, gave 6 grains per gallon, of which 2 grains consisted of organic matters. The well at Thames Head, sunk in the Great Oolite near Cirencester, yielded water containing 16 grains per gallon. And the waters of the Chelt, near Cheltenham, which rise from springs at the base of the inferior Oolite, gave 20 grains per gallon, of which 4 grains consisted of organic matter.
(e) New Red Sandstone.—Next to the Chalk, the New Red Sandbtone, including the Bunter and Lower Kenper divisions, is the most important water-bearing formation, and the water which it yields possesses an advantage over that of the Chalk in being softer, and generally capable of being used for all domestic and manufacturing purposes. From the numerous a,nalyses that have been made of these waters in different localities in the central and north-western counties, we have the means of arriving at general conclusions on this subject.
The beds of the Bunter Sandstone aro wonderfully adapted to act both as natural filters and as reservoirs for that portion of the rain which sinks below the surface. This may be assumed as one third, on an average, of the actual rainfall; while in some districts—where the formation consists of soft sandstone, or unconsolidated conglomerate, devoid of a thick covering of drift clay—the amount of absorption must reach well-uigh one-half the amount of the rainfall. Owing also to its uniformity of composition, and the absence of beds of clay or marl of any importance, the whole mass of rock below a certain level, and throughout a depth of several hundred feet in some districts, becomes water logged ; and wells sunk therein do not, as in the case of the Chalk, generally depend for their supply on the presence of fissures, water being nearly always found after the " water-level " of the immediate district has been reached.