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GEOLOGY The rivers are of course sustained by permanent springs, but owing to pollution from ditches, land drains, crude sewage from villages, or the effluents of sewage farms, the water is entirely unfit for domestic use. Reservoirs for the supply of canals are situated at Byfield, Braunston, Daventry, Welford and Naseby ; these are fed chiefly by the Marlstone, though Drift beds contribute. The reservoir for supplying Northampton is situated in the valley between Ravensthorpe and Teeton, it is fed by springs from the Northampton Sand, drains about 3,000 acres, and covers about 400 acres when full. Kettering reservoir is situated in a valley near to Thorpe Malsor ; it is fed chiefly by Northampton Sand springs. The majority of the waters are rather hard, hence so-called petrify- ing springs are fairly numerous. Chalybeate waters occur too at a number of places, no doubt due to the presence of ferrous sulphate as a product of the decomposition of iron pyrites. A red deposit around a spring may be quite local in origin (see p. 31) the main body of water feeding the spring being practically free from dissolved iron. MINERALS Besides the essential minerals of the clays, sands and limestones of the various rocks already considered, certain other minerals, or modifica- tions of the essential ones, occur ; these are briefly referred to below. Brown Haematite, Limonite, or Hydrated Ferric Oxide is the chief form of iron-ore, whether from the Northampton Sand or the Marlstone, etc. Carbonate of Iron is the mineral constituting the green or grey rock, or cores of some brown rock, in the Northampton Sand. Silicate of Iron only occurs as a colouring matter to the green and blue varieties of ironstone. Phosphate of Iron occurs as a colouring matter in the ironstones, and as a bright blue incrustation, vivianite, on objects in the River Alluvium. Specular Iron, a form of haematite, is found occasionally as dark lustrous crystals in some Lias nodules, or even in fossils. Iron Pyrites occurs in a variety of forms — small moss-like masses in the Armatus zone of the Lower Lias ; small rods in the Pettos zone ; characteristic cubes in the Middle Lias, upper part of Upper Lias, and lower part of the Northampton Sand (Raunds) ; large irregular lumps in the upper part of the Upper Lias (Paulerspury, Grafton Regis, etc.) ; radiated pyrites in the Northampton Sand (Wellingborough). Manganese Protoxide occurs to a small extent in most ironstones. Calcium Carbonate is found in many forms — as nail-head spar (cone-in-cone), chiefly in the middle beds of the Upper Lias, where very large nodules occur, the entire outer part of which is this mineral ; dog- tooth spar in cavities of limestone rock, and very commonly where coral has been ; lenticular crystals occasionally ; Jibrous calcite (beef, so-called) 39